COMPARATIVE ANATOMY 



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

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

 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 pro- 

 vided 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 appear- 

 ance to a mulberry, is spoken of as a morula. 



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



C D 



Fig. 2. DIAGRAMS OF THE SEGMENTATION OF THE OOSPERM. 



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

 C, further stage. D, morula stage. 



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

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

 enclosing this cavity form the germinal membrane or blasto- 

 derm. Consisting at first of a single layer of cells, the blastoderm 

 later on becomes two- and then three-layered. From the relative 

 positions of these, they are spoken of respectively as the outer, 

 middle, and inner germinal layers, or as epiblast, (ectoderm,) 

 mesoblast, (mesoderm,) and hypoblast (endoderm). 



An increase in the amount of food-yolk (deuteroplasm, see note on 

 p. 3) present in the ovum results in certain modifications of the primi- 

 tive process of segmentation as described above. Yolk is an inert 



