October 23, 2017History of Medicine
In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.
Aristotle's On the Generation of Animals is referred to in Latin as De Generatione Animalium. As with many of Aristotle's writings, the exact date of authorship is unknown, but it was produced in the latter part of the fourth century BCE. This book is the second recorded work on embryology as a subject of philosophy, being preceded by contributions in the Hippocratic corpus by about a century. It was, however, the first work to provide a comprehensive theory of how generation works and an exhaustive explanation of how reproduction works in a variety of different animals. As such, De Generatione was the first scientific work on embryology. Its influence on embryologists, naturalists, and philosophers in later years was profound. Among these were Hieronymus Fabricius, William Harvey, St. Thomas Aquinas, and Charles Darwin.
A brief overview of the general theory expounded in De Generatione requires an explanation of Aristotle's philosophy. The Aristotelian approach to philosophy is teleological, and involves analyzing the purpose of things, or the cause for their existence. These causes are split into four distinct types: final cause, formal cause, material cause, and efficient cause. The final cause is what a thing exists for, or its ultimate purpose. The formal cause is the definition of a thing's essence or existence, and Aristotle states that in generation, the formal cause and the final cause are similar to each other, and can be thought of as the goal of creating a new individual of the species. The material cause is the stuff a thing is made of, which in Aristotle's theory is the female menstrual blood. The efficient cause is the mover or what causes the thing's existence, and for reproduction Aristotle designates the male semen as the efficient cause. Thus, while the mother's body contains all the material necessary for creating her offspring, she requires the father's semen to start and guide the process.
De Generatione consists of five books, each containing multiple chapters. Books I and II are of most interest to embryology. Book III is a comparative study of zoology that applies principles from Book II to distinct species of animals. Book IV contains miscellaneous information about aspects of reproduction, such as how heredity works and birth defects occur. Book V compares the characteristics that all animals share, and is primarily a discussion of sensory organs and the physical appearance of animals, focusing on characteristics like hair, coloration, voice, and teeth. Aristotle's research and writing, influenced Renaissance scholars. Early studies of embryology came from the work of the Italian anatomists Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardan, Emilio Parisian, Fortunio Licata, Stefano Lorentzian, Spallanzani, Enrico Sertoli, and Mauro Rossini.
Editor's note: Dear Readers, it's not clear to us, how Aristotle's theories of epigenesis became temporarily derailed by the ideas of preformation. We're guessing it was the prevailing notions of the church. If any reader has more specific knowledge, we would appreciate an email, clearing up this approximately, 150-year gap, when Aristotle's ideas were eschewed.
By the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo - a preformed, miniature human, or homunculus - that was planted in the female during intercourse, which then grew into a larger being, as it developed during pregnancy. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. However, during the late 18th century an extended and controversial debate among biologists finally led epigenesis to eclipse the short-lived, but established preformationist view. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.
After 1827: Karl Ernst von Baer and Heinz Christian Pander proposed the germ layer theory of development; von Baer discovered the mammalian ovum in 1827. Modern embryological pioneers include Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, August Weismann, Gavin de Beer, Ernest Everett Just, and Edward B. Lewis.
After the 1950s, with the DNA helical structure being unraveled knowledge increased in the field of molecular biology, and developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated. So, here in the 21st Century, we can acknowledge the genius of Hippocrates and Aristotle, whose correct observations many centuries ago, prevail. Those ancient Greeks knew a thing or two. Sources: Wikipedia; nih.gov; asu.edu.