COMPARATIVE ANATOMY 



is 



the 



both male and female germinal cells. This structure 

 morphological expression of the characters of the species. 



After fertilisation has taken place development begins. The 

 segmentation nucleus divides into two equal parts, each of which 

 forms a new centre for the division of the oosperm, as it must 

 now be called, into two halves or Uastomcres. This division, the 

 beginning of the process of segmentation, takes place by the 

 formation of a furrow round the egg which becomes deeper and 

 deeper until the division is complete. (Fig. 2, A). 



The first stage in the process of segmentation is thus com- 

 pleted ; the second takes place in exactly the same way, and 



FIG. 2. DIAGRAMS OF THE SEGMENTATION OF THE OOSPERM. 



A, first stage (two segments) : JfK, polar cells. B, second stage (four segments). 



C, further stage. D, morula stage. 



results in a division of the oosperm into four parts ; and by a similar 

 process are formed eight, then sixteen, then thirty-two blastomeres, 

 and so on, the cells becoming smaller and smaller, and each being 

 provided with a nucleus. (Fig. 2, c D). In short, out of the 

 original oosperm a mass of cells is formed which represents the 

 building-material of the animal body, and which, from its likeness 

 in appearance to a mulberry, is spoken of as a morula. 



In the interior of the morula a cavity (segmentation cavity or 

 llastoccele) filled with fluid is formed, and the morula is now spoken 

 of as the blastosphere or blastula (Fig. 3). The peripheral cells 

 enclosing this cavity form the germinal membrane or blasto- 



