160 GENERAL BIOLOGY 



is a rounded mass of tiny cells that looks somewhat 

 like a mulberry, and, for that reason, the stage is 

 often called the morula. In very few species, how- 

 ever, is the process of cleavage so regular and sym- 

 metrical as just described. In many forms, not 

 only are the cells of a different size, but the cleavages 

 do not all occur in regular order, some coming on 

 faster than others. In nearly all cases, however, 

 the result of the continued cleavage is to produce 

 a mass of cells that remain attached to one another 

 in a single layer. Owing perhaps to the stress of 

 surface tension acting on the cells and pulling them 

 away from the center, the resultant is a hollow sphere. 

 The stage just described is called the blastula. The 

 closed cavity surrounded by the single layer of cells 

 (blastomeres) is called the blastoccele. 



In eggs in which there is a large amount of yolk, 

 the cells resulting from cleavage are of very unequal 

 size. Since the yolk is heavier than the protoplasm, 

 it " settles " to one side of the zygote, and the 

 blastomeres that arise on that side are therefore 

 larger on account of the presence of the yolk, and, 

 for the same reason, slower in forming than those 

 of the opposite side. This reaches its extreme in 

 eggs like those of birds, in which the protoplasm is 

 crowded to a small disk-like spot at one side, and the 

 mass of yolk is so great that the cleavage is not accom- 

 plished at all, but affects only this protoplasmic 

 disk-like spot. Such an egg is termed meroblastic 

 or partially cleaving, in contrast to holoblastic or 

 totally cleaving, eggs. 



