Chapter 1
Historical aspects of embryology
Key Points
- Up to the eighteenth century, the prevailing view of many scientists and scholars interested in embryology was that of preformation, namely that organisms develop from miniatures of themselves.
- An alternative hypothesis of embryonic development, referred to as epigenesis, proposed that the structure of an animal emerges gradually from a relatively formless egg. The epigenesis theory, first proposed by the Greek philosopher Aristotle, preceded the preformation theory by two millennia.
- Major advances in reproductive and developmental biology took place in the seventeenth century. Until that time, early civilisations held the view that a foetus resulted from the mixing of two parental āseedsā.
- In human embryology, ovists believed in generation from oocytes while spermists believed that males contributed the essential characteristics of their offspring with females contributing only a material substrate. This theory was the dominant view of embryonic development until the late seventeenth century.
- As microscopy improved during the eighteenth century, biologists observed that embryos developed in a series of progressive steps and epigenesis displaced preformation as the basis of embryological development.
- Progress in understanding and manipulation of reproductive biology from a point in the past when the origins of human life were not understood to a point where early embryos can be generated in vitro represents a phenomenal scientific achievement.
Introduction
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 āgenerationā, 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 āreproductionā 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.
Dominant theories of generation in the seventeenth and eighteenth centuries
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 origins of life
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, ālike bred likeā, 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 BronisÅaw 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 āfatherā literally means āmy motherās husbandā, 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 āseedā, 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 maleās role in the creation of new life rapidly became dominant. For example, in the Book of Genesis, it is written that Onan āspilled his seed on the groundā in order to avoid making his sisterāinālaw pregnant. In Egyptian mythology, the story of creation relates that AtumāRa created the earth, and the first god and goddess, from his seed through masturbation. This semen/seed analogy dominated all subsequent thinking about generation.
Contributions of the Ancient Greeks
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 femaleās 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 maleās semen or āseedā contributed to the āformā of the foetus and that this form was imprinted onto the āmatterā 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 womenās 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...
