36 CATEGORIES OF CLASSIFICATION. 



mal, Baer opened to us a new field of investi- 

 gation in the embryology of the four types, 

 showing that for each there was a special mode 

 of growth in the egg. Looking at them from 

 this point of view, we shall see that these four 

 types, with their four modes of growth, seem to 

 fill out completely the plan or outline of the 

 animal kingdom, and leave no reason to expect 

 any further development or any other plan of 

 animal life within these limits. The eggs of all 

 animals are spheres, such as I have described 

 them; but in the Radiate the whole periphery is 

 transformed into the germ, so that it becomes, 

 by the liquefying of the yolk, a hollow sphere. 

 In the Mollusks, the germ lies above the yolk, 

 absorbing its whole substance through the under 

 side, thus forming a massive close body instead 

 of a hollow one. In the Articulate, the germ 

 is turned in a position exactly opposite to that 

 of the Mollusk, and absorbs the yolk upoji the 

 back. In the Vertebrate, the germ divides in 

 two folds, one turning upward, the other turning 

 downward, above and below the central backbone. 

 These four modes of development seem to ex- 

 haust the possibilities of the primitive sphere, 

 which is the foundation of all animal life, and 

 therefore I believe that Cuvier and Baer were 

 right in saying that the whole animal kingdom 

 is included under these four structural ideas. 



