THE PERIOD OF DESCRIPTIVE EMBRYOLOGY 5 



and formless beginning to a complex organization characteristic of 

 the adult. Basing this observation on his study of the hen's egg, he 

 laid the foundation for the modern concept of epigenesis or unfold- 

 ing development. This concept is opposed to the idea of structural 

 preformation of the embryo within the gamete, or germ cell. 



William Harvey (1578-1657) long ago came to the conviction 

 that all animals arise from eggs: "Ex ovo omnis"; and later Virchow 

 (1821-1902) went even further to state that all cells are derived 

 from preexisting cells: "Omnis cellula e cellula." Flemming stated: 

 "Omne vivum ex nucleo" and later Huxley said, "Omne vivum ex 

 vivo." These concepts, taken for granted today, are basic in biology 

 and emphasize the fact that only through the process of reproduction 

 has the present population of organisms come into existence. Embry- 

 onic development is a prerequisite to survival of the species. There- 

 fore, only those organisms that survive their embryonic development 

 and achieve the stage of reproductive ability can carry the baton of 

 protoplasm from the previous to the future generations in the relay 

 race of life with time. 



Fabricius (1537-1619) and Harvey were both limited in their 

 studies by the lack of the miscroscope but both of them presented re- 

 markable studies of early chick development. Malpighi (1628-94), 

 using the newly invented microscope, gave us two works in em- 

 bryology: "De Formatione Pulli in Ovo" and "De Ovo Incubato," 

 which deal largely with the development of the chick from 24 hours 

 to hatching. Beginning at this stage he was misled to believe that all 

 the various parts of the embryo are preformed within the egg (since 

 chick embryos incubated for 24 hours exhibit most of the major organ 

 systems) and that the process of development was one simply of 

 growth and enlargement. This was the theory of "preformationism" 

 which is diametrically opposed to that of epigenesis or unfolding de- 

 velopment. This new concept of preformationism found a staunch 

 supporter in Swammerdam (1637-80). In 1675 Leeuwenhoek firmly 

 believed he discovered the human form sitting in a cramped position 

 in the head of the human spermatozoon. This led to the "spermist" 

 school of preformationists. 



It was inevitable that, when parthenogenesis was discovered, the 

 spermists would have to retire in favor of the ovists (e.g.. Bonnet) 

 since eggs were known to develop into organisms without the aid of 

 spermatozoa. This led to the equally ridiculous concept of the "em- 



