286 BIOLOGY. [BOOK in. 



This preparatory labour once accomplished in the ovulum, the 

 process of segmentation makes way before the process of genesis. 

 The cells of the blastoderm gradually disappear in proportion 

 as spring up among them by genesis the anatomical elements 

 destined to constitute definitively the new individual. It is 

 between the two blastodermic vestments that arise these first 

 elements. Thenceforth the difference is marked between the 

 invertebrated animals and the vertebrated animals. In these 



FIG. 41. 



First stadium of the evolution of a mammifer, " segmentation of the ovum," multiplica 

 tion of the cells by reiterated scissions : A, the ovum divides by a first fissure into two 

 cells ; B, the two cells divide into four cells ; C, these last divide into eight cells ; D the 

 segmentation, indefinitely reiterated, has produced a spherical mass of numerous cells. 



last, in effect, we soon see traced the rudiment of the vertebral 

 column, the cells of the notocord (Fig. 42). This notocord en- 

 sheaths itself. Then we behold the formation of the first cartila- 

 ginous bodies of the vertebrates, and of the first elements of the 

 central nervous axis ; and so on. All the tissues and all the 

 organs thus appear little by little. At the moment of their 

 birth the anatomical elements are already for the most part well 

 characterised. Nevertheless they undergo another evolution; 

 they develop themselves ; azotised granulations, nucleoles, are 

 formed little by little in their substance ; their volume augments. 

 But, once arrived at their complete development, they are stable, 

 or at least no longer modify themselves, except regressively, to 

 disappear and perish. Never is an animal histological species 



