202 BIOLOGY: GENERAL AND MEDICAL 



the poles of the nuclear spindle always being directed 

 toward the greatest protoplasmic masses, so that there 

 result four, eight, sixteen, thirty-two, sixty-four, 128 

 cells, or blastomeres, and so on. This process of cleavage, 

 though taking place after karyokinetic changes, differs 

 from ordinary cell division in that it progresses so rapidly 

 that no time is allowed for growth and the cells become 

 smaller and smaller as they divide. 



In equal cleavage the cells of the two poles are of 

 uniform size; in unequal cleavage their number is the 

 same, but the size varies, the animal cells being smaller 

 than the vegetative cells. Equality of numbers is not, 

 however, preserved, for either the richness of protoplasm 

 in the animal half or the magnitude of the cells of the 

 vegetative half determines that the former outgrows 



SECTION OF MORULA. 



SECTION OF BLASTULA. 



Archiccele. 



FIG. 82. Section of morula. (Masterman.) 

 FIG. 83. Section of blastula. (Masterman.) 



the latter until with 128 cells in the animal half there 

 may be but thirty-two in the vegetative half, and so on. 



The inequality of the cleavage is in direct proportion 

 to the quantity of yolk at the vegetative pole. Thus, 

 in holoblastic eggs, the cleavage may be equal; in the 

 enormous meroblastic eggs of birds the yolkless proto- 

 plasm assembled at the animal pole is alone able to un- 

 dergo segmentation, and the primitive assemblage of cells 

 resulting from this localized cleavage appears as a cellu- 

 lar disc floating upon the surface of the yolk. This 

 germinal disc receives the name " blastoderm." 



The segmentation first results in the formation of a 

 solid cellular mass which bears a partial resemblance 

 to a mulberry and is known as a morula. Every egg 

 passes through this stage. Soon, however, the morula 



