AND CLASSIFICATION. 29 
differently from the Mollusk,—the Mollusk dif- 
ferently from the Radiate. Cuvier only showed 
us the four plans as they exist in the adult ; Baer 
went a step further, and showed us the four plans 
in the process of formation. 
But his greatest scientific achievement is per- 
haps the discovery that all animals originate from 
eggs, and that all these eggs are at first identical 
in substance and structure. The wonderful and 
untiring research condensed into this simple 
statement, that all animals arise from eggs, and 
that all those eggs are identical in the beginning, 
may well excite our admiration. This egg con- 
sists of an outer envelope, the vitelline membrane, 
containing a fluid more or less dense, and various- 
ly colored, the yolk; within this is a second en- 
yelope, the so-called germinative vesicle, contain- 
ing a somewhat different and more transparent 
fluid, and in the fluid of this second envelope 
float one or more so-called germinative specks. 
At this stage of their growth all eggs are micro- 
scopically small, yet each one has such tenacity 
of its individual principle of life that no egg was 
ever known to swerve from the pattern of the 
parent animal that gave it birth. 
