252 FOUNDATIONS OF BIOLOGY 



have shown, this involves cleavage of the egg, followed, in the 

 Metazoa, by bias tula and gastrula stages during which the 

 primary germ layers are established the fundament out 

 of which the definitive form, organs, and organ systems of the 

 adult are evolved. The description and comparison of these 

 processes in different organisms constitute the content of one 

 aspect of EMBRYOLOGY. We must be satisfied merely with the 

 realization of the fact that animal development, though it 

 varies widely in producing the immensely diverse body forms, 

 exhibits throughout a thread of similarity in its broader 

 fundamental features. (Figs. 19, 69.) 



But embryology is something more than the description 

 of the kaleidoscopic series of stages which seem to melt one 

 into the other as development progresses. It attempts, espe- 

 cially at the present time, to look below and beyond structure 

 to the processes involved, and to determine how the sequence 

 of events is brought about. This is but a repetition of the 

 stages of progress in all science; a passage from the descrip- 

 tive to the experimental. Although many of the results thus 

 far secured are necessarily largely tentative, they have gone 

 far toward placing the science of biology as a whole on an 

 experimental basis. 



From what the pioneer students of embryology during the 

 seventeenth and eighteenth centuries saw, or thought they 

 saw, with simple lens and newly invented compound micro- 

 scope, there were gradually formulated two opposing views 

 of development which, though long since swept aside in their 

 original form as a result of the increase of knowledge, raised 

 the problem of problems that is still before the embryologist 

 to-day. In brief, one view virtually denied development by 

 maintaining that the adult organism is nearly or completely 

 formed within the germ, either in the egg or the sperm, which 

 merely by expansion, unfolding, and growth gives rise to the 



