FROM SIMPLE CELL TO COMPLEX ANIMAL 37 



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.D. 



53. Forms of Segmentation. The conditions suggested above give rise to the 

 following classes of segmentation. 



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. 



13,4. 



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. (Illustrated in many mollusks and in the 

 Amphibia.) Fig. 13, 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. 13, C. 

 II. Peripheral: in which an excess of yolk collects at the centre of the ovum, 

 with the protoplasm at the periphery. The dividing nuclei assume a 

 superficial position and surround the unsegmented yolk. (Illustrated in 

 the eggs of insects and other arthropods.) Fig. 13, D. 



