AND CLASSIFICATION. 29 



differenfiy 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- 

 velope, the so-called gcrminative 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. 



