3 2 ZOOLOGY. 



female pronucleus and fuses with it; thus is formed the first 

 segmentation nucleus. With the addition of the chromosomes 

 in the male nucleus the fertilized ovum contains the same 

 number of chromosomes as before maturation, which in each 

 species of animals is a constant number. It appears that fer- 

 tilization restores to the female cell essentially what it lost in 

 the process of maturation, and in addition stimulates it to 

 active nuclear and cytoplasmic division as indicated in the next 

 paragraph. Follow the process in Fig. 10. 



50. Segmentation or Cleavage. Following shortly upon 

 fertilization, if conditions are favorable, ordinary mitotic 

 nuclear division begins and the ovum divides promptly into 

 2, 4, 8, 16, etc., cells (blastomeres). The resulting cells be- 

 come smaller and smaller with each division, since the whole 

 egg-mass does not increase appreciably in size meanwhile. 

 The first three cleavage planes are usually perpendicular to 

 each other. Their position is much modified, however, by 

 the presence of food or yolk substance in the egg. The yolk 

 in general retards cleavage. If the yolk is in small quantity 

 and is uniformly distributed through the egg, the blastomeres 

 will be about equal in size (Fig. n, A), and will continue to 

 divide with practically equal promptness. If there is much 

 of the yolk it is not likely to be uniformly distributed. Under 

 the influence of gravity and internal forces, the yolk is likely to 

 collect at the lower, and the protoplasm and nucleus at the up- 

 per pole of the ovum (Fig. 1 1, B, C). The protoplasmic pole 

 is known as the active or formative pole and the lower, as the 

 passive or nutritive pole. The polar bodies are normally freed 

 at the formative pole. Under these circumstances the blasto- 

 meres at the nutritive pole are larger and divide less rapidly 

 than those in which the protoplasm is in excess. If the 

 yolk is excessive in amount that portion of the ovum in which 

 it collects may be totally prohibited from dividing. 



51. Forms of Segmentation. The conditions suggested above give 

 rise to the following classes of segmentation. 



