EMBRYOLOGY I 8 1 



distinctly two legs, thighs, breasts and arms. ... it was a delightful and 

 incredible sight." * (Emphasis should be, of course, on the word, incredible.) 

 Here we have a bold description of preformation as envisaged by an ar- 

 dent spermist. Other observers thought they saw microscopic horses in the 

 semen of the horse, similar animalcules, but with larger ears, in the semen 

 of the donkey, and minute roosters in the semen of the rooster. 



The ovists were no less backward than the spermists in putting forward 

 their claims for the egg. "Emboitment," the notion that, like box within 

 box, all future structures were already present within the egg, presented 

 no particular obstacle for a dyed-in-the-wool preformationist of the ovist 

 school. As one (Haller) wrote: "It follows that the ovary of an ancestress 

 will contain not only her daughter, but also her grand-daughter, her great- 

 granddaughter, and her great-great-granddaughter; and if it is once proved 

 that an ovary can contain many generations, there is no absurdity in saying 

 that it contains them all." 



Naturally, the preformationists did not flourish unchallenged. Opposing 

 theories grew up under the general title of epigenesis. In its extreme form 

 epigenesis was the exact opposite of preformation. The theory of epigenesis 

 held that the egg, for example, was a simple homogeneous structure. Ac- 

 cording to this view, there was not only no preformation, but, on the 

 contrary, no differentiation at all within the egg or spermatozoon. Ad- 

 herents of the most extreme views of epigenesis were under the necessity, 

 therefore, of explaining how heterogeneity could come from homogeneity, 

 how an undifferentiated egg could give rise to the complicated differ- 

 entiated structures of embryo and adult. And, as so frequently happens, 

 when obstacles such as these are encountered, recourse was often made to 

 some type of vital force, a vis essentialis or a nisiis formativus, which it was 

 easy to think, shaped the course of embryological events. 



Embryologists have long recognized that neither the extreme view of 

 preformation nor epigenesis were correct. Yet, there were elements of 

 truth in each. The spermatozoon and the egg, each is far from being a 

 homogeneous structure. Each is a highly differentiated cell, with compli- 

 cated internal organization. Still, the differentiation of the spermatozoon, 

 the egg and the developing embryo are in no sense simply the presence in 

 a miniature of adult structures. Development of the fertilized egg is a far 

 more complicated process than any early embryologist ever dreamed. And, 

 only in relatively recent years has the embryologist been able to glimpse 

 some of the underlying mechanisms involved. 



Descriptive embryology has long been concerned in following the 

 origin of adult structures back to their earliest appearances in egg and 

 embryo. Thus we have been able to recognize "anlagen" or primordia of 



* Dalenpatius as quoted by Vallisneri. From Lewis and Stohr, Text-Book of Histol- 

 ogy, Blakiston, 1913. (English translation by Lewis from Berger's German translation 

 of Vallisneri.) 



