EMBRYOLOGY 



97 



the lowly Protozoa and Protophyta to the vertebrates and 

 flowering plants. 



Not only do we find that there are these two kinds of 

 series, the anatomical and the embryological, but we find 

 that the two series often correspond to a remarkable degree. 

 Take an illustration. Among the vertebrates, fishes are the 



FIG. 19. Stages in the development of the pond snail (Lymn&us). [After HAECKEL.] 



simplest on the whole. The Amphibia are in general some- 

 what more modified in their organization. The reptiles and 

 birds are still more so, and the mammals are in some re- 

 gards the most highly developed of all. Now, as we study 

 the embryology of the Mammalia, we find that in some 

 features of their general organization and in the character of 

 many of their separate organs the different stages in their 



