INTRODUCTION 



boitement" or "encasement" theory, which suggested that each egg 

 contains, in miniature, all the future generations to be developed 

 therefrom. Each generation was achieved by shedding the outermost 

 layer, like Chinese ivory boxes carved one within another. But pre- 



formationism of either school was des- 

 tined to lose adherents as the microscope 

 became further refined. Caspar Friedrich 

 Wolff (1733-94) attacked the idea of pre- 

 formationism and supported epigenesis 

 on purely logical grounds, put forth in 

 his "Theoria Generationis." In 1786 he 

 published the most outstanding work in 

 the field of embryology prior to the works 

 of von Baer. It was entitled "De Forma- 

 tione Intestinorum" and in this treatise 

 Wolff showed that the intestine of the 

 chick was developed de novo (epigeneti- 

 cally) out of unformed materials. 



The Period of Comparative Embryology 



Up to about 1768 embryology was al- 

 most exclusively descriptive and morpho- 

 logical. It was inevitable that the second 

 phase in the history of embryology would 

 soon develop, and would be of a compar- 

 Spermist's conception of the ative nature. This approach was stimu- 

 human figure in miniature j^^^^ ^ q^^-^^^ (1769-1832) and his 

 within the human sperm. (Re- , . .• . t 



J r^ /^ Tj * • f emphasis on comparative anatomy. In 



drawn after O. Hertwig, from ^ ^ ■^ 



Hartsoeker: 1694.) 1824 Prevost and Dumas first saw cleav- 



age or segmentation of an egg in reptiles. 

 In 1828 von Baer published his "Entwicklungsgeschichte der Tier" 

 and thereby became the founder of embryology as a science. He estab- 

 lished the germ layer doctrine, proposed a theory of recapitulation, 

 and made embryology truly comparative. From a study of the devel- 

 opment of various animals, von Baer arrived at four important con- 

 clusions, which are known collectively as the laws of von Baer. They 

 are as follows: 



1 . "The more general characteristics of any large group of animals appear 

 in the embryo earlier than the more special characteristics." 



