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TEXT-BOOK OF EMBRYOLOGY. 



Comparing maturation in the male and female sex cells (Fig. 16), it is to be 

 noted that the spermatogonia and oogonia proliferate by ordinary mitosis, 

 maintaining the somatic or diploid number of chromosomes up to a certain 

 period in their life history. They then enter upon a period of growth in size, 

 resulting in primary spermatocytes and primary oocytes (Fig. 16). When 

 these prepare for division the nuclear reticulum in each case resolves itself into 

 the haploid number of chromosomes. During division this reduced number is 

 given to each resulting secondary spermatocyte or oocyte. 



There is, however, this marked peculiarity about the division of the primary 

 oocyte, that while the division of the nuclear material is equal the division of the 

 cytoplasm is very unequal, most of the latter remaining in one cell, the secondary 



FiG. 15. From section of ovum (primary oocyte) of the mouse, showing first maturation 

 spindle. Note the 12 chromatin segments, the somatic number of chromosomes being 24. The 

 ovum is surrounded by the zona pellucida (z.p.) and the corona radiata. Sobotta. 



oocyte proper. The other cell, very small owing to its lack of cytoplasm, is 

 extruded from the oocyte proper as the first polar body (Fig. 16). The same 

 condition obtains in the next division. One cell, the mature ovum, retains 

 most of the cytoplasm, the other being detached as the second polar body (Fig. 

 16). In some cases the first polar body also divides. Thus the primary oocyte 

 gives rise to three or four cells, each of which has the reduced number of chromo- 

 somes. One of them becomes the mature ovum, the others are cast off as 

 apparently useless and eventually die. The primary spermatocyte, on the other 

 hand, gives rise to four functioning cells which are equal in cytoplasmic as well 

 as in chromatin content (Fig. 16). 



The apparent difference between maturation of the male and female sex 



