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 upon 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. 
