Chapter 1 Embryology, as it relates to domestic animals, is concerned with the sequential stages of embryonic and foetal development, beginning with fertilisation. This dynamic science utilises cell biology, genetics and biochemistry to explain the complexities of development. All mammals begin life as embryos. Despite the steadily increasing understanding of embryonic development and its underlying regulatory mechanisms, much remains to be discovered. For students of animal biology, veterinary medicine and related health sciences, embryology offers an insight into the development of the mammalian body at both the microscopic and anatomical levels. It also provides an important introduction to animal genetics, organ systems and reproductive biology. At a superficial level, the basis of human reproduction is widely understood in most modern societies. In previous centuries, however, biological aspects of reproduction in the human population and among animal populations were a cause of considerable debate and much uncertainty prevailed. In the seventeenth and eighteenth centuries, the issue of ‘generation’, as the formation of new life was called, evoked strong religious and philosophical responses on the part of theologians and scholars, generating more heat than light. Indeed, the term ‘reproduction’ was not used until the eighteenth century. Prior to that time, there was no understanding that an organism was being copied, as the term implied. In the history of embryology, preformationism was a theory of generation widely accepted from the late seventeenth to the end of the eighteenth century. This concept proposed that organisms develop from miniature versions of themselves, already fully formed in the eggs or sperm of their parents prior to conception. Epigenesis, the alternative theory to preformationism, contended that through a series of stages each embryo or organism was gradually produced from an undifferentiated mass. Ovism, which held that the maternal egg was the location of the preformed embryo, was one of two models of preformationism. The other model, known as spermism, contended that offspring develop from a tiny, fully formed, embryo contained within the head of a sperm. The origin of spermism derived from the microscopic demonstration of the existence of sperm in the late 1670s. Support for ovism peaked in the mid to late eighteenth century but, by the turn of the nineteenth century, it had declined. While spermism was never as dominant as ovist preformationism, it had ardent followers whose work and writings greatly influenced the development of embryology during this period. The art forms which were a feature of Stone Age civilisations conveyed the thinking of the time in relation to generation. Some of the earliest images created by humans are Venus figurines carved from soft stone, bone or ivory, or made of fired clay, most of which date from the Gravettian period, 28,000 to 22,000 years ago. In some of these figurines, certain parts of the female anatomy including the abdomen, hips, breasts, thighs and vulva were exaggerated. Archaeologists speculate that these figurines may be fertility symbols and may represent the earliest images of humans endeavouring to understand their own biological origins. Prior to the seventeenth century, assumptions relating to the origin of life varied. It was generally believed that in mammals, including humans, ‘like bred like’, although it was not certain that this always occurred. Some believed, for example, that women could give birth to other species; claims that an English woman, Mary Toft from Godalming, Surrey, gave birth to rabbits in 1726 were widely accepted before she confessed that her story was untrue. As recently as the beginning of the last century, the Polish anthropologist Bronisław Kasper Malinowski (1884 to 1942) claimed that the inhabitants of the Trobriand Islands in the South Pacific were unaware that babies resulted from sexual intercourse. In their native language, the word for ‘father’ literally means ‘my mother’s husband’, suggesting a social rather than biological relationship. While perhaps surprising at first, there are good reasons why a link between the sexual act and the birth of a child may not have been obvious, since women can have sex without becoming pregnant. Furthermore, even when conception did occur, the two events, sex and birth, were separated by 9 months and were therefore not immediately associated with each other. Indeed, it has been postulated that human understanding of the association between mating and reproduction came through the domestication of animals some 10,000 years ago. In these animals, mating only occurs during a defined period of sexual receptivity termed oestrus, creating an observable link between mating and pregnancy. On the basis of these observations, the realisation that male semen or ‘seed’, the only clearly and immediately observable product of copulation, was fundamental to the creation of life became central to the concept of generation. In religious beliefs and in mythology, the male’s role in the creation of new life rapidly became dominant. For example, in the Book of Genesis, it is written that Onan ‘spilled his seed on the ground’ in order to avoid making his sister‐in‐law pregnant. In Egyptian mythology, the story of creation relates that Atum‐Ra created the earth, and the first god and goddess, from his seed through masturbation. This semen/seed analogy dominated all subsequent thinking about generation. In Europe, up to the second half of the seventeenth century, beliefs on virtually every question relating to life science were dominated by the teaching of Ancient Greek philosophers. In the fifth century BCE, the Greek physician Hippocrates (circa 460 to 370 BCE), considered to be one of the most outstanding figures in the history of medicine, argued that generation took place through the joint action of two kinds of semen, one from the male ejaculate, the other from the female’s menstrual blood. A century later, the Greek philosopher and scientist Aristotle (384 to 322 BCE) published De generatione animalium (The Generation of Animals) about 350 BCE, the first work to provide a comprehensive theory of the mechanisms of reproduction in a variety of animals. He described the concepts of oviparity (birth from eggs), viviparity (live birth) and ovoviviparity (production of an egg that hatches inside the body). He also described the holoblastic and meroblastic patterns of cell division (see Chapter 5). He made the important observation that the organs develop gradually in the embryo (epigenesis) and are not preformed. In contrast to Hippocrates, Aristotle believed that only the male’s semen or ‘seed’ contributed to the ‘form’ of the foetus and that this form was imprinted onto the ‘matter’ which was provided by the menstrual blood of the female, much like a seal stamping hot wax. Another analogy, which has persisted to the present day, was that semen was like a seed which was sown on fertile ground. Aristotle argued that lower animals such as insects generated spontaneously from decay. This theory corresponded with the everyday experience of observing maggots appearing suddenly on rotting matter, but this concept was ultimately refuted by Francesco Redi (1626 to 1698) in the mid 1600s (see below). In the second century CE, Galen (129 to circa 200), a prominent Greek physician, surgeon and philosopher in the Roman Empire, supported the assertion of Hippocrates that the seeds of both the male and female contribute to procreation. This was partly due to his mistaken view that women’s genitalia were identical to those of men but turned inward. His anatomical reports, based mainly on dissection of monkeys and pigs, remained uncontested until printed descriptions and illustrations of human dissections were published in 1543 in the classical work on human anatomy De humani corporis fabrica (On the Fabric of the Human Body) by the Belgian anatomist and physician Andreas Vesalius (1514 to 1564). Although Galen adopted Hippocrates’ view that there were two types of ‘semen’ – one male, the other female – acceptance of this theory was hampered by the fact that it was not possible to identify female semen and therefore Aristotle’s view persisted. For several hundred years, controversy persisted as to the respective roles of the male and female in generation. During this time, the ideas of the Ancient Greeks were maintained by Arab thinkers but were not developed beyond those focusing on the role of the male and, for a period, progress in understanding the origin of life did not occur. From the fourteenth century onwards, there was a resurgence in Europe of the ideas of the ancient thinkers. Around this time, among the famous anatomical drawings of the great Italian artist Leonardo da Vinci (1452 to 1519) were those of the pregnant bicornuate bovine uterus and of the foetus and foetal membranes with the uterus removed. Da Vinci also depicted a human uterus opened to reveal the foetus and associated membranes. One of the first major publications on comparative embryology was De formato foetu (The Formed Foetus) in 1600 by the pioneering Italian anatomist and surgeon Hieronymus Fabricius (Girolamo Fabrizio da Acquapendete, 1537 to 1619) which contained many illustrations of embryos and foetuses at different stages of development. Fabricius was a student of Gabriele Falloppio (Fallopius, 1523 to 1562) who described the uterine tubes, formerly referred to as the Fallopian tubes. The site of B lymphocyte formation in birds, the bursa of Fabricius, now known as the cloacal bursa, bore his name. A manuscript entitled De formatione ovi et pulli (On the Formation of the Egg and Chick), found among his lecture notes after his death, was published in 1621 and contained the first description of the bursa. Another Italian, Bartolomeo Eustachius (1514 to 1574) published illustrations of canine and ovine embryos in 1552. He also extended the knowledge of the anatomy of the internal ear by describing correctly the auditory tube that connects the middle ear with the nasopharynx, and which bears his name (the Eustachian tube). Many of the concepts associated with embryology were speculative until the invention of the microscope, which allowed detailed observation of embryological structures. Marcello Malpighi (1628 to 1694), an Italian professor of medicine and personal physician to Pope Innocent XII, was one of the first supporters of preformationism. He described development of the embryo as a mere unfolding of an already miniature adult organism. He published the first microscopic examination of chick embryo development in 1672, identifying the neural groove, the somites and blood flow to the yolk sac. Because of the importance of his early work, a number of anatomical structures were named after him, including Malpighian (renal) corpuscles in the kidney and the Malpighian layer in the epidermis. He observed that even the unincubated chick egg was considerably structured, leading him to question the concept of epigenesis and to believe that a preformed version of the chicken resided in the egg. These observations were subsequently questioned, as his ‘unincubated’ eggs had in fact been left exposed to warm environmental temperatures. Nonetheless, these experiments opened up one of the great debates in embryology: whether the organs of the embryo formed de novo at each generation (epigenesis) or were already present in miniature form within the egg or sperm (preformation). A period of intense discovery in the seventeenth century laid the foundations for the unravelling of sex, life and growth and for our current knowledge on the origins of life. It was during this period that fundamental discoveries relating to biological events associated with procreation were made, although their full meaning remained unclear. William Harvey (1578 to 1657), a one‐time student of Falloppio and personal physician to King James I and King Charles I, best known for his discovery of blood circulation, undertook one of the first detailed investigations in embryology. In 1651 Harvey published his book Exercitationes de generatione animalium (On the Generation of Animals) with the now famous frontispiece illustrating the Greek god, Zeus, liberating all creation from an egg bearing the inscription ‘ex ovo omnia’ (all things come from the egg) (Fig 1.1). Harvey was convinced that the egg, rather than sperm, was fundamental to generation, apparently challenging Aristotle’s belief that sperm were of greatest importance, although what exactly he meant by ‘egg’ is unclear. He had no understanding of there being equivalent male and female gametes and no idea of what might be contained in semen. In the 1630s Harvey carried out a now famous experiment in which he dissected the deer of King Charles I during rutting and mating. He found no trace of semen in the uterus, nor did he find any changes in the female ‘testicles’, the generally accepted term at the time for what we now call ovaries. In addition, he failed to recognise the filamentous conceptus characteristic of ruminants. He ultimately concluded that Aristotle was correct and that semen acted in some way by shaping menstrual blood. Figure 1.1 The frontispiece of William Harvey’s Exercitationes de generatione animalium, published in 1651, showing Zeus liberating all living things from an egg bearing the inscription ‘ex ovo omnia’ (magnified on right). Courtesy of Wellcome Library, London. At the University of Leiden in the 1660s three medical students, Nicolas Steno (Niels Stensen, 1638 to 1686), Jans Swammerdam (1637 to 1680) and Regnier de Graaf (1641 to 1673) made a significant impact on our knowledge of generation. All three were heavily influenced by their professor, Johannes van Horne (1621 to 1670) and, in the case of Swammerdam and Steno, by the French author and scientist Melchisedec Thévenot (1620 to 1692). Both scientists encouraged the three students to investigate generation and the origins of life. In 1667, Steno, who by this time was in Florence, published what turned out to be his most influential scientific work, Elementorum myologiae specimen (A Model of Elements of Myology), in which he accurately described the function of muscles, using both dissection and mathematical models. He included a comparison between the anatomy of the viviparous dogfish and of egg‐laying rays and concluded, based on his observations, that the ‘testicles’ of women were analogous to the ovaries of the dogfish. Swammerdam initially focused on the generation of insects and, through careful observation and dissection, he came to the radical conclusion that all animals derive from eggs laid by females of the same species. In his 1669 book, Historia generalis insectorum, he put forward a revolutionary classification of insects based on their modes of development which is still in use. Together with the work of the Italian biologist Francesco Redi, Swammerdam’s study showed that insects did not generate spontaneously, as had previously been thought, but were the product of an egg laid by a female of the same species and that the same organism persists through various stages, namely larva, pupa, juvenile, adult. Redi refuted the notion of spontaneous generation by demonstrating, through simple experimentation, that maggots appearing on decaying matter came from the eggs of flies. His most famous experiments are described in Esperienze intorno alla generazione degl’insetti (Experiments on the Generation of Insects), published in 1668. In 1671, de Graaf published a brief outline of his work, in which he summarised his view of how ‘eggs’ in the female ‘testicle’ became ‘fertile’ through the action of the ‘seminal vapour’ rising up from the uterus via the uterine tubes. In 1672, he published De mulierum organis generationi inservientibus tractatus novus (New Treatise Concerning the Generative Organs of Women
Historical aspects of embryology
Introduction
Dominant theories of generation in the seventeenth and eighteenth centuries
The origins of life
Contributions of the Ancient Greeks
The emergence of comparative embryology
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