SEXUAL AND ASEXUAL REPRODUCTION 



253 



an immense amount of food 

 number of chromosomes 

 that are found in the ordi- 

 nary cells of the animal. 

 This doubling of the chro- 

 mosomes is the last step in 

 the formation of the oocyte. 

 Maturation of the Egg. 

 At the stage shown in Figure 

 121 A, the egg is not yet 

 mature, i. e., is not yet ready 

 to unite with the sperm; it 

 must first pass through a 

 further series of changes 

 spoken of as the maturation 

 of the egg. The nucleus ap- 

 proaches the edge of the 

 egg and divides into two 

 parts, one very large and 

 one very small, each retain- 

 ing four of the chromosomes 

 present in the original nu- 

 cleus; Fig. 121 B-D. It will 

 be noticed that in this divi- 

 sion the chromosomes do 

 not split* as they do in or- 

 dinary cell division (see 

 page 87), but that each 

 of the two nuclei formed 

 contains half of the original 

 eight. One of these nuclei 

 is pushed out of the egg 

 as a small protrusion 

 shown at D; the other one 

 a short period of rest the 



yolk in it, and with double the 



J 



FIG. 121. DIAGRAM SHOWING THE 

 MATURATION AND FERTILIZATION OF 



AN EGG 



Stages A to G represent maturation; H and 

 I the fertilization; J, the egg after it has di- 

 vided; /, female pronucleus; ra, male pronu- 

 cleus; p, polar bodies; sp, a typical flagellate 

 sperm more highly magnified. 



remains within the egg. After 

 process of division is repeated, 



