FROM SIMPLE CELL TO COMPLEX ANIMAL 37 



Thus is formed the first segmentation nucleus, and the egg is 

 fertilized. Wi^th the addition of the chromosomes in the male 

 nucleus the fertilized ovum contains the same number of chro- 

 mosomes as before maturation, which in each species of animals 

 is a constant number, except for the differences between the 

 sexes. It appears that fertilization restores to the female cell 

 essentially what is lost in the process of maturation, and in 

 addition serves to stimulate it to active nuclear and cytoplasmic 

 division as indicated in the next paragraph. 



52. 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) . In the first division the chromosomes 

 split, and one-half of each chromosome brought in by the sperm 

 nucleus and one-half of each furnished by the egg-nucleus go to 

 each daughter nucleus. This is continued in later divisions, and 

 in this way every cell of the body gets its chromatic material 

 equally from the father and the mother. 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. 13, 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 upper, pole of the 

 ovum (Fig. 13, 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 blastomeres at the nutritive pole 

 are larger and divide less rapidly than those in which the proto- 

 plasm is in excess. If the yolk is excessive in amount that por- 

 tion of the ovum in which it collects may be totally prohibited 

 from dividing as will be seen in Fig. 13, C,I>. 



