DEVELOPMENT. 29 



union of male and female nuclei restores the reduced 

 number of chromosomes, which remain constant and 

 usually even in number for every succeeding division. 

 In certain insects an odd number of chromosomes 

 appears, in which case the embryo develops into a 

 male. By repeated divisions a spherical mass of 

 cells is produced, known as the morula stage. 



Fig. 6. Blastula of triton tseniatus: jfi, segmentation cavity; rz, mar- 

 ginal zone; dz, cells with abundant yolk (Hertwig). 



Blastula. The spherical mass quickly develops 

 into a hollow sphere, lined by a single layer of cells, 

 and is then a blastula. The cavity of the blastula 

 is the segmentation cavity. 



Gastrula. A more vigorous growth seems to 

 take place at one point of the blastula, producing 

 lateral pressure and an invagination at that place 

 so as to form a two-layered cup-like structure in 

 some eggs a blastoderm known as the gastrula. The 

 gastrulae vary considerably according to the different 

 forms of cleavage. It is an established fact that all 

 metazoa pass through the morula, blastula, and 



