The human mating process encompasses the social and cultural processes whereby one person may meet another to assess suitability, the courtship process and the process of forming an interpersonal relationship. productive success - so mating psychology has been a massive &39;target&39; of the selective process. J B Ogle Animal Shelter Animal Shelters City, Village & Township Government County & Parish Government Website (812) 282-0071 201 Willinger Ln Jeffersonville, IN 47130 CLOSED NOW 5. They are responsible for the production of sperms and the male hormone testosterone. The stud should be bathed thoroughly before mating. Some flying will ants bite, depending on their species. processes that guide the mating preferences of men and women are identical, and no sex-dif ferentiated predictions can be derived. Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping. Mar 14, 2018 &183; We offer bearded dragons for sale year round. These strategies include long-term mating, short-term mating, extra-pair mating, mate poaching, and mate guarding. When the dog and bitch lock together, this is called a tie mating. . As their descendants, modern humans have. Hot Science - Cool Talks 86Dr. In this article, we&39;ll explore the biology of sex -- otherwise known as human sexual reproduction. When it&x27;s time to mate, up to 100 males can go after a single female at the same time; the female ends up in the middle of a large mating ball. Access to high-quality food is the limiting factor for these tropical birds. 65 in Sexual Selection and the Origins of Human Mating Systems). Your account has no avatar. 24K subscribers Subscribe 820 6. The sperm and the ovum are known as the gametes (each. Since humans do have a mating call, all one must do is listen. It&x27;s thought that at one time, human ancestors did engage in chimp-like habits of sex and child-rearing, in which strong alpha males mated freely with the females of their choice, and then left the. Video clip id 1014399902. Human mating process live The process of reproduction in humans usually begins with copulation, followed by the Pre-fertilization, Fertilization, and Post-fertilization. For a mammal with no official mating season, its surprising that the majority of births occur between July and September. During this fundamental process, both male and female reproductive organs play an important role. Also since the time it takes for human babies to mature enough to no longer need parental support is longer than 1 year, itd be irrational to have an annual season for mating. Mating Season (Natsu x reader lemon Modern) Alibaba x reader Part 2. Find professional MatingHumanvideos and stock footage available for license in film, television, advertising and corporate uses. Do not feed the stud before mating as there are chances that he might vomit in the process of mating. Then the male takes the females bra off and kisses her breast. Hot Science - Cool Talks 86Dr. In addition to fast, swooping motions, Costa&x27;s hummingbirds look to impress mates with a more horizontal dance, swaying their bodies from side to side while flexing their brightly colored throat patches. If the reproductive act is complete, sperm cells are passed from the male body into the female, in the process fertilizing the females egg and forming a new organism. The Giant breeds do not become sexually mature until they are 6 to 9 months of age. sexual intercourse, also called coitus or copulation, reproductive act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract. During sexual intercourse, the interaction between the male and female reproductive systems results in fertilization of the ovum by the sperm. See human mating stock video clips Image type Orientation Color People Artists More Sort by Popular Vegetables Plants Biology Icons and Graphics Business People Egg Sperm 3d rendering Fertilization Human pregnancy of 121. They have one offspring every couple of years. The glans at the head of the penis swells and the testes enlarge in the male. Next, she carefully fluffs and arranges the many covers. yd; yo. September is the most common birth month in the US, according to one. Explain the term trimester system In Biology, the trimester system mainly refers to three months. Monogamy, when required, polygamy when possible, is ideal for humans. Statistics is a crucial process behind how we make discoveries in science, make decisions based on data, and. Find professional Mating Human videos and stock footage available for license in film, television, advertising and corporate uses. The 2nd stage of fluid has the most sperm in it and it is released during vigorous thrusting. As winged ants fly looking for mates, some. This film was inspired by a friend&x27;s description of a night out. Love is woman&x27;s moon and sun. The human desire for companionship is one of the strongest human drives. The latest research suggests they lived side-by-side with modern humans (Homo sapiens) for up to 5,000 years before their final disappearance, with the two species of humans interbreeding quite frequently. This is the process, take care to realize, if you refuse to engage in all these steps you stop the process of mating. Since humans do have a mating call, all one must do is listen. Therefore, a socially-acceptable sexual partner must be defined (1). Filmed on Tuesday 15th January 2019. Kneading for Possible Mates. They respond to her scent and the call. Find professional MatingHumanvideos and stock footage available for license in film, television, advertising and corporate uses. With Tenor, maker of GIF Keyboard, add popular Stallion Mating With Human animated GIFs to your conversations. Once the male finds a suitable mate, he bites into her belly and latches on until his body fuses with hers. Sexual intercourse, or copulation, is the deposition of sperm into a female via a male intromittent organ. In the male, blood flows into the penis, causing it to become erect; in the female, the vaginal walls become moist, the inner part of the vagina becomes wider, and the clitoris enlarges. In most cases, it is the female who initiates the mating process as soon as she ready. The ejaculation is so powerful that it ruptures the endophallus, disconnecting the drone from the queen. dadvi making breakfast with his kids. 65 in Sexual Selection and the Origins of Human Mating Systems). "Taking you and your people to places never thought possible. Human males will attract mates by getting a lot of power, either through physical or economical means. Objectives This study was designed to comprehensively explain the theoretical view of the human mating process. Category Fun & Crazy Added on 09 Nov 2018. The estrus cycle lasts for 36 days and during that time, the female experiences major changes in the shape, size, and color of the genital skin. David BussSeptember 6, 2013Presented by the Environmental Science Institute as part of Hot Science - Cool Talks. - mating human stock videos & royalty-free footage. 1 2 People and animals that sexually reproduce use sexual intercourse to have an offspring. If they are from the garden, you will recognise these as the ones you peed on yesterday. Human males will attract mates by getting a lot of power, either through physical or economical means. In the feral cat world cats mating gives the impression of being the equivalent of a human gang bang. You need to understand, they are an organization with. The first phase was a cross-sectional quantitative study with two Farsi-modified versions of instruments preferences concerning potential mates and factors of choosing a mate; content. mw parser output. Choose from a wide range of similar scenes. In the plateau stage, breathing becomes more rapid and the muscles continue to tense. Cultural elements, especially religion, foster a trend toward social monogamy. The first stage is a clear fluid that has practically no sperm in it. Find professional MatingHumanvideos and stock footage available for license in film, television, advertising and corporate uses. Find Human Mating stock images in HD and millions of other royalty-free stock photos, illustrations and vectors in the Shutterstock collection. 5em 1em 1em background f8f9fa border 1px solid aaa padding 0. Single Plot. Pregnancy requires a lot of energy, after all, so theres little point for a deer, for example, to try and have a child in the middle of winter when food is at its scarcest. Human reproduction naturally takes place as internal fertilization by sexual intercourse. 28 feb 2022. They are responsible for the production of sperms and the male hormone testosterone. Before I jump into the process I want to explain WHY this is important information for your spiritual journey. Here, a total of 135 geese were selected, specifically, Sichuan white geese (Anser cygnoides), Zhedong white geese (Anser cygnoides), and Hungarian geese (Anser anser) (300-day-old), and the mating behavior was monitored daily from 600 a. Cultural elements, especially religion, foster a trend toward social monogamy. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features Press Copyright Contact us Creators. levi with a son headcanons. Filmed on Tuesday 15th January 2019. Here, a total of 135 geese were selected, specifically, Sichuan white geese (Anser cygnoides), Zhedong white geese (Anser cygnoides), and Hungarian geese (Anser anser) (300-day-old), and the mating behavior was monitored daily from 600 a. I launched "The NFT Podcast" on Youtube in March 2021. The two actually share a circulatory system; the male receives nutrients from the female, and the female receives sperm from the male. It is a random process. Mating is the process of sexual union between a male and female. Dog mating mating. This paradox then necessitates that women (and by default men) must subscribe to short term and long term schedules of mating. Humans often mate with those resembling themselves , a phenomenon described as positive assortative mating (PAM). Read Mating season from the story Dsmp x reader one shots by hawkswife4052 with 1,035 reads. yd; yo. His work. Sexual intercourse. mating strategies that led to their forebears success. Big Black Horse with Girls - 2021 Horse Mating Videos Animal Breeding Interesting WorldSince the inception of our Buffalo breeding program, we have specia. mating strategies that led to their forebears success. Most lizards require a male and female pairing to mate; the male excretes an odor from a gland on the tail to which the female is attracted. Sperm displacement. Get a 25. Human sexual intercourse 1. The human mating process encompasses the social and cultural processes whereby one person may meet another to assess suitability, the courtship process and the process of forming an interpersonal relationship. With all of the negative comments, I too was skeptical at adopting from Main Line Rescue. A magnifying glass. If you need a refresher on the birds and the bees, this is the article for you. who is already in a relationshipwhat can be called the process of human mate. sexual intercourse, also called coitus or copulation, reproductive act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract. productive success - so mating psychology has been a massive &39;target&39; of the selective process. While approaching the fertile female, the male will adopt an imposing and haughty posture, arching his neck slightly to. Dog mating mating. Mating is the process of sexual union between a male and female. Goofy is an animated character that first appeared in 1932's Mickey's Revue. Dog mating mating. This shows that human sexual behavior is unusually flexible, since in most animal species, one mating system dominates. Image type. sexual intercourse, also called coitus or copulation, reproductive act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract. This process is experimental and the keywords may. 1 During sexual intercourse, the interaction between the male and female reproductive systems results in fertilization of the ovum by the sperm to. Human Mating Watched 4,286 times Report a bug 57. When it&x27;s time to mate, up to 100 males can go after a single female at the same time; the female ends up in the middle of a large mating ball. Log In My Account vg. David BussSeptember 6, 2013Presented by the Environmental Science Institute as part of Hot Science - Cool Talks. Sex The Most Important Human Behaviour. The rarer phenomenon of negative assortative mating (NAM. when levi had a crush. 3 of Kensington&39;s parody of Planet Earth Planet Humans - The Mating RitualYou could call Proverbs God&39;s Book of Tweets. 4. Kimberton, PA 19442. The process of reproduction in humans usually begins with copulation, followed by the Pre-fertilization, Fertilization, and Post-fertilization. Thousands of new, high-quality pictures added every day. 1 2 People and animals that sexually reproduce use sexual intercourse to have an offspring. They are responsible for the production of sperms and the male hormone testosterone. For the most part, humans and other large primates likely aren&x27;t driven to reproduce because of specific seasonal cues, says Melissa Emery Thompson, an anthropologist at the University of New Mexico. Sesshomaru grids against me rubbing my. It typically involves sexual intercourse between a sexually mature human male and female. It is the houses of the testicles, or testes, and. Even Stranger Forms of Sexual Intercourse Dolphins engage in a sexual intercourse that is very similar to that of humans. It is the houses of the testicles, or testes, and. Gestation period. - mating human stock videos & royalty-free footage. Log In My Account vg. Count back the months and youll realize that these babies were. Hummingbird species that live in the tropics generally do not migrate. Thousands of new, high-quality pictures added every day. sexual intercourse, also called coitus or copulation, reproductive act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract. Dec 20, 2012 Sex is a part of life and a subject of scientific research. They live in their feral forms all the time, dwelling in the mountains and forests with real wolves. As is often the case with humans, a female pigeon&x27;s sex drive is higher than a male&x27;s. There is a single species, made up of a set of interbreeding organisms. What are the. Find Human mating stock images in HD and millions of other royalty-free stock photos, illustrations and vectors in the Shutterstock collection. () Humans clearly have some concept of a mating season as a set of months in which people prefer to start a pregnancy, and that season depends on many factors such as the temperature and the change in temperature. 5 ago 2020. 14 Did you like this video Yes No Tags Fun Funny People Parody Join other players talking about games Try CryptoServal Game NFT game backed by Y8. Chapter 6. navigation Jump search Courtship behavior humans human mated pair. Garden snails. The animals will form a so-called mating herd, in which up to a dozen males will follow and huddle around a single female to mate with her. The male reproductive system consists of Testicles (testes) A pair of oval-shaped organs masked in a pouch called the scrotum. Mating is a natural process that does not always occur with the intention of producing offspring. In humans, the intromittent organ is the penis, and sperm are deposited into the vagina, where they make their way to towards the uterus to fertilize an egg. A woman ovulates one egg during her monthly menstrual cycle, and the egg lives for only 24 hours. As should be obvious, the video is sexually explicit. Thousands of new, high-quality pictures added every day. 17 abr 2017. We&39;ll examine the body&39;s sexual organs, the biological cycles of sex and the process of fertilization. Then the male takes the females bra off and kisses her breast. As if the two facts above aren&x27;t strange enough, turns out there is a "she-male" snake who releases pheromones just like the females do (and fools hundreds of other males to pile up on himher). See human mating stock video clips Image type Orientation Color People Artists More Sort by Popular Vegetables Plants Biology Icons and Graphics Business People Egg Sperm 3d rendering Fertilization Human pregnancy of 121. Never feel confused in Human sexuality class again Our short 5-minute videos explain complicated Human sexuality concepts in a manner that&39;s easy for you to understand. Spring Street Southwest, Huntsville, Madison County, United States. . 19 may 2016. J B Ogle Animal Shelter Animal Shelters City, Village & Township Government County & Parish Government Website (812) 282-0071 201 Willinger Ln Jeffersonville, IN 47130 CLOSED NOW 5. Thousands of new, high-quality pictures added every day. () Humans clearly have some concept of a mating season as a set of months in which people prefer to start a pregnancy, and that season depends on many factors such as the temperature and the change in temperature. Animal mating seasons typically coincide with the season where resources (namely food) are most abundant. When she is ready to lay her eggs, months down the road in some species, she uses the sperm to fertilize them. Pairing the Dogs. Explain the term trimester system In Biology, the trimester system mainly refers to three months. It is an innate feature of human nature and may be related to the sex drive. Alan Dixson is your go-to on this. The bright side is that research on human mating. Egg hatching. Pregnancy requires a lot of energy, after all, so theres little point for a deer, for example, to try and have a child in the middle of winter when food is at its scarcest. This film was inspired by a. Megan Stubbs, another sexologist who lives in Michigan, agrees. With all of the negative comments, I too was skeptical at adopting from Main Line Rescue. What are the. Our mating season is all the time except for one week a month. "What this means is that even people in the Upper Palaeolithic, who were living in tiny groups, understood the importance of avoiding inbreeding . Microscopic mites have been present in a person&x27;s body from the day they were born. We will probably never know for certain, but the scenario that researchers consider to be most plausible is as follows. Male animals go through precise mating movements, without rehearsing them. They give birth at most once every 5 years, and the interval between babies can be as long as 10 years. However, no pregnancy can result from their copulation. Horse Mating With Female Horse Animals Mating Like Humans For Real 6httpyoutu. Kneading for Possible Mates. Equally important is the fact that the twin data challenge the fundamental assumption that mate selection is a strategic process. Female hummingbirds choose their mates based on a couple of factors, including the. It grounds human behavior in evolutionary biology. We&39;ll examine the body&39;s sexual organs, the biological cycles of sex and the process of fertilization. We can feed ourselves any time. The two actually share a circulatory system; the male receives nutrients from the female, and the female receives sperm from the male. Explain the term trimester system In Biology, the trimester system mainly refers to three months. Our mating season is all the time except for one week a month. While there are close analogies between animal mating systems. In the process, he cut his hand on the bloody knife. Just to put this into perspective, if a human man had a proportionately long penis, he could use the carpool lane. Search Lil Baby Net Worth Net Baby Lil Worth altur. Human mating psychology is complicated. The mating antics of humans are, well, odd. Plots for the burial of infants or children generally cost less. Reproduction Process of Human infographic. Humans also pick up pheromones and chemosignals from potential mates through olfaction. Buss University of Texas at Austin Department of Psychology Abstract Modern humans have inherited the mating strategies that led to the success of their ancestors. Mating may also lead to external fertilization, as seen in amphibians, fishes and plants. Count back the months and youll realize that these babies were. Daniel Conroy-Beam uses an evolutionary perspective to understand how mate preferences are linked to . The urethra traverses the corpus spongiosum, and its opening, the meatus (miets), lies on the tip of the glans penis. The amplexus embrace between frogs stimulates the female to release her eggs which she expels in the form of a gelatinous mass. We&39;ll examine the body&39;s sexual organs, the biological cycles of sex and the process of fertilization. What are the. The first mating induces ovulation in the female cat and so subsequent matings will happen to ensure that she has been fertilized. Well, we don&39;t really mate we have sex. Download footage now. In 2010, researchers. Mating rituals Concept art of mating. He&x27;s conservative in his assessment of the literature on penis size and even he concedes that human penis "circumference is unusual when compared to the penes of other hominoids (apes)" (p. September is the most common birth month in the US, according to one Harvard study, with September 16th being the most common day. Videos of vaginal sex (5 C) Categories Human sexual activity. 2022 Author altur. timoshenko beam calculator, garmin chartplotter
Human reproduction is sexual reproduction that results in human fertilization to produce a human offspring. . Human mating process live
It&x27;s thought that at one time, human ancestors did engage in chimp-like habits of sex and child-rearing, in which strong alpha males mated freely with the females of their choice, and then left the. Humans aren&39;t monogamous like most animals. The mid-size breeds become sexually mature and ready for action at 4 to 4 months. In the 1 990s the new science of evolutionary psychology made valiant attempts. Find professional Mating Human videos and stock footage available for license in film, television, advertising and corporate uses. The reproduction process can be full of rituals for the Monkey. Some dogs get panicked and that locking position may take longer because of the induced anxiety, while some studs stay calm and get. The pain is necessary for feline mating as it is the shock to her system that. . Bed bugs generally live for 12 to 18 months. Human Mating Watched 4,294 times. It is the houses of the testicles, or testes, and. Thousands of new, high-quality pictures added every day. When those just-right human cells were injected into the pig embryos, the embryos survived. 28 feb 2022. The human mating process encompasses the social and cultural processes whereby one person may meet another to assess suitability,. The first humans emerged in Africa around two million years ago, long before the modern humans. Aug 06, 2020 Evidence has long been accumulating that humans and Neanderthals mated while their populations overlapped in Europe, before Neanderthals went extinct around 30,000 years ago. "They do lunges and they will come together and do swats and even try to grab onto the nape of each other&x27;s necks. Log In My Account dj. The human female reproductive cycle repeats every month instead of every year. Regardless of the exact sexual stimulation, the underlying pattern is comparable in both sexes. In the process, he cut his hand on the bloody knife. Think about the brilliant plumage of peacocks, the bulky antlers of. Filmed on Tuesday 15th January 2019. beIh07fh8fOpU Subscribe httpswww. Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping together. 95 views 4 days ago. Sexual intercourse, or copulation, is the deposition of sperm into a female via a male intromittent organ. 4. Getty Images offers exclusive rights-ready and premium royalty-free analog, HD, and 4K video of the highest quality. dadvi x reader on vacation. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features Press Copyright Contact us Creators. Males may have to share though, as one female can fuse with many males. Breeding is well planned and depends on the social behaviour and maturity of the organisms. Children living with one genetic and. It usually entails an explicit or implicit social . That group is not human, but. Humans aren&39;t monogamous like most animals. There are total of 4 types of speciation i. Just to put this into perspective, if a human man had a proportionately long penis, he could use the carpool lane. Some chemicals have been investigated for pheromone actions in humans but evidence is. to 600 p. The human mating process encompasses the social and cultural processes whereby one person may meet another to assess suitability, the courtship process and the process of forming an interpersonal relationship. "Taking you and your people to places never thought possible. 11,642 human mating stock photos, vectors, and illustrations are available royalty-free. Added to your profile favorites. Now, in The Mating Mind, a pioneering work of evolutionary science, these aspects of human nature are at last explored and explained. 4M views 15 years ago I don&39;t understand how these humans ever propagate the species. Sexual intercourse (or simply called sex) is the insertion and thrusting of a male&39;s penis into a female&39;s vagina. Males are called bulls and they are ready to mate when they are about 9 or 10 years of age. The Giant breeds do not become sexually mature until they are 6 to 9 months of age. fc-falcon">Sexual intercourse. But in time she allows him closer and closer. Added on 09 Nov 2018 Comments Please register or login to post a comment Register Login. Sep 02, 2015 Horse Mating With Female Horse Animals Mating Like Humans For Real 6httpyoutu. The tone is a series of melodious sounds meant to attract a potential mate. Pregnancy requires a lot of energy, after all, so there&x27;s little point for a deer, for example, to try and have a child in the middle of winter when food is at its scarcest. In this article, we&39;ll explore the biology of sex -- otherwise known as human sexual reproduction. It really depends on the individual dogs and their breed. Your account has no avatar. It is when two humans of the opposite sex love each other so much and they have had sexual relationships and they want to have more, the man goes nude and the woman wears a thong and a really push up bra and they sit in bed. In the feral cat world cats mating gives the impression of being the equivalent of a human gang bang. Most lizards require a male and female pairing to mate; the male excretes an odor from a gland on the tail to which the female is attracted. () Humans clearly have some concept of a mating season as a set of months in which people prefer to start a pregnancy, and that season depends on many factors such as the temperature and the change in temperature. Mating behavior is a critically important component of poultry reproduction. Human Mating Watched 4,286 times Report a bug 57. It is only observable in pubescent teens. And always much thanks to you, my readers. Made with Lionhead Studios (A. Mating is the action of animals coming together to breed or produce more offspring and is a necessary process in all species that reproduce sexually. It typically involves sexual intercourse between a sexually mature person with testes and a person with ovaries. The first group is human and female (the "daughters of humankind"). Find Human mating stock images in HD and millions of other royalty-free stock photos, illustrations and vectors in the Shutterstock collection. The male reproductive system consists of Testicles (testes) A pair of oval-shaped organs masked in a pouch called the scrotum. There is such a thing as a human mating dance, or in other words, specific courtship moves that humans go through, just like animals do. Apr 01, 2000 This chapter traces theoretical and empirical progress in the study of human mating over the past few decades. Find professional MatingHumanvideos and stock footage available for license in film, television, advertising and corporate uses. From The Nature. We discuss how existing theories fail to incorporate trade-offs to explain fully within-sex variation in mating tactics. This process is experimental and the keywords may. I offer legal consulting support on commercial matters and solutions, combining purpose and values with plain language drafting and visual design to create clear, collaborative and human centric legal documents. This is generally in the smaller breeds such as the Polish Dwarf and the Dutch. There is one. "Males don&39;t have tusks or claws or anything like that. There are 3 stages of a dog&x27;s ejaculate. Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping. Move thru DSP. I launched "The NFT Podcast" on Youtube in March 2021. every step of the mating process, from mate selec-. Sexual intercourse. yd; yo. 9 of women. Why do human and non-human species conceal mating. Download footage now. 17 abr 2017. It is very likely that the entire mating process is actively. But humans aren&39;t the only species who have to put up with some inconveniences. It views human nature as a set of biological adaptations, and tries to discover which problems of living and reproducing those adaptations evolved to solve. The first group is human and female (the "daughters of humankind"). Objectives This study was designed to comprehensively explain the theoretical view of the human mating process. descriptions off, selected; Captions. Many researchers have considered mate choice the main operator in human sexual selection. "Males don&39;t have tusks or claws or anything like that. Man has other forms of fun. Female cats have an additional reason for kneading. 18 jun 2014. A male will force about 50 females together on a beach as his harem, and will mate with them while biting them to keep them from fleeing. Animal mating seasons typically coincide with the season where resources (namely food) are most abundant. Find Human Mating stock images in HD and millions of other royalty-free stock photos, illustrations and vectors in the Shutterstock collection. Sometimes sexual intercourse is called coitus or copulation and is more casually known as having sex or sleeping. Human sexual intercourse 1. Egg dropping. Big Black Horse with Girls Trainer - Horse Mating With Humans For Real Interesting World - YouTube 000 1610 Big Black Horse with Girls Trainer - Horse Mating With Humans For Real. Some dogs get panicked and that locking position may take longer because of the induced anxiety, while some studs stay calm and get. Animal mating seasons typically coincide with the season where resources (namely food) are most abundant. Policy Management. When she is ready to lay her eggs, months down the road in some species, she uses the sperm to fertilize them. This strategy is characterized by Darwin&39;s . They are responsible for the production of sperms and the male hormone testosterone. . ib physics syllabus 2023 pdf