120 METAZOA IN GENERAL 



lower pole, leaving the cytoplasm as a disc at the upper pole. In this 

 case the egg cell is known as telolecithal (Fig. 49 A). In such egg cells 

 the upper pole is called the animal pole; and the lower, the vegetal pole. 

 Primitive mammals have telolecithal egg cells, and the ancestors of 

 mammals doubtless had such egg cells. 



In the insects a third arrangement is presented. Here the yolk 

 occupies the central portion of the egg cell, inclosing within it, at the 

 very center, the nucleus, which is surrounded by some of the cytoplasm, 

 while the greater part of the cytoplasm forms a layer over the whole 

 surface. Such an egg cell is called centrolecithal (Fig. 49 C). Here there 

 is a considerable amount of yolk, though relatively not so much as in 

 telolecithal egg cells. 



145. Forms of Cleavage. — The process which follows fertilization 

 and which results in the formation of a large number of cells from the 

 fertilized egg cell is known as cleavage. The individual cells which are 

 thus formed are termed hlastomeres. 



The effect of the difference in the amount and distribution of the 

 yolk is seen in the different ways in which egg cells cleave. In an ideal 

 embryogeny, which may be accepted as that of a homolecithal egg cell, 

 the plane in which the first cell division takes place is typically meridional, 

 passing from one pole to the other. The second cleavage plane is also 

 meridional, being at right angles to the first, and results in the formation 

 of four similar cells. The third cleavage plane, however, passes at right 

 angles to the two others, and thus eight cells are produced. Of these the 

 upper four will be smaller than the lower four. Since the whole egg has 

 been involved in the cleavage, the egg is sometimes termed holohlastic and 

 the cleavage is called total. If the yolk is quite evenly distributed and 

 the cells which result from the cleavage are all approximately the same 

 size, it is termed equal cleavage (Fig. 48 B). If, however, the yolk is not 

 evenly distributed but is greater toward the lower pole of the egg cell, the 

 cells at the upper pole will be decidedly smaller than the other four. This 

 is termed unequal cleavage (Fig. 48 D and E). In some cases the two 

 cells first formed differ m size, the first cleavage plane being horizontal 

 and the smaller cell being above the larger. In other cases the difference 

 in the size of the cells does not develop until the third cleavage occurs, the 

 upper four being smaller than the lower. There are many modifications 

 of these different types. 



In telolecithal and centrolecithal egg cells the cleavage planes do not 

 pass entirely through the cell but only through the cytoplasmic portion, 

 and thus the cleavage becomes partial; such egg cells are often termed 

 merohlastic. 



In telolecithal egg cells the division of the cytoplasm results in an 

 embryonic disc at the animal pole, and accordingly the cleavage is termed 

 discoidal (Fig. 49 J5). As development proceeds and the cells continue 



