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. II, 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. u, 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, 2). 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 rilled with fluid. The cavity is termed the segmen- 

 tation cavity (Fig. ir, 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 imagination, and the product as a 

 gastrula (Fig. u, 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, an4 one within called 

 entoderm or hypoblast (Fig. 11,4). 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 



