34 ZOOLOGY. 



A. Total segmentation. 



I. Equal : in which there is little yolk material, and that is well 

 distributed. (Illustrated in most of the lower invertebrates 

 and mammals.) Fig. n, A. 



II. Unequal : in which there is a moderate amount of yolk which 

 accumulates at the passive pole. The cells at the active pole 

 are more numerous and smaller than at the passive. (Illus- 

 trated in many mollusks and in the amphibia.) Fig. n, B. 



B. Partial segmentation. 



I. Discoidal : in which there is an excessive amount of yolk, with 

 the nucleus and a small mass of protoplasm occupying a disc 

 at the active pole. This disc alone segments, and the embryo 

 lies upon the yolk. (Illustrated in the eggs of fishes, birds and 

 reptiles.) Fig. n, C. 



II. Peripheral : in which an excess of yolk collects at the centre 

 of the ovum, with the protoplasm at the periphery. The divid- 

 ing nuclei assume a superficial position and surround the unseg- 

 mented yolk. (Illustrated in the eggs of insects and other 

 arthropods.) Fig. n, D. 



52. Blastula and Morula. As cleavage continues the 

 blastomeres remain associated in a spherical mass. The in- 

 dividual cells project beyond the general surface not unlike 

 the lobes of a mulberry, and for this reason this stage is called 

 the morula or mulberry stage (Fig. n, <?). By the growth 

 of the cells and by the imbibition of water the morula may 

 become a hollow sphere of cells (blastula) the central cavity 

 of which is filled with fluid. The cavity is termed the segmen- 

 tation cavity (Fig. n, s.c). 



53. Gastrula. In those eggs in which the segmentation is 

 total, a next important step is the pushing in of that side of 

 the blastula which corresponds to the original nutritive pole. 

 The process is known as invaginution, and the product as a 

 gastrula (Fig. n, 4). It takes place much as one might sup- 

 pose one side of a hollow rubber ball to be dimpled or infolded 

 by the exhaustion of the air within. The gastrula is to be de- 

 scribed as made up essentially of two layers of cells, one ex- 

 ternal and called ectoderm or epiblast, and one within called 

 entoderm or hypoblast (Fig. n, 4). The segmentation cavity 

 may be wholly obliterated ; in that case the entoderm and ecto- 

 derm come to lie in contact. The cavity of the invagination 



