MATURATION. 19 



polar body constitute secondary oocytes, comparable with the secondary 

 spermatocytes of the male. The egg now leaves the ovary and reaches the 

 oviduct. If a sperm enters the ovum, another spindle forms and a second 

 polar body is given off. The nucleus of the mature ovum or female pronu- 

 cleus, with the haploid number of chromosomes, is now ready for union 

 with the male pronucleus. (See Fig. 1 1 .) 



Comparing maturation in the male and female sex cells, it is to be noted 

 that the spermatogonia and oogonia proliferate by ordinary mitosis, main- 

 taining 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, result- 

 ing in primary spermatocytes and primary oocytes. 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 oocyte proper. The other cell, very small owing to its 

 lack of cytoplasm, is extruded from the oocyte proper as the first polar body. 

 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. 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 chromosomes. One of them becomes the mature ovum, the others are 

 cast off as apparently useless cells and eventually disappear. The primary 

 spermatocyte, on the other hand, gives rise to four functioning cells which 

 are equal in cytoplasmic content. (See Fig. 12.) 



The apparent difference between maturation of the male and female sex 

 cells the single functional cell in the female as contrasted with four in the 

 male loses some of its character when one notes that in some forms the 

 polar bodies are not so rudimentary as is generally the case. Thus in certain 

 forms one or more of the polar bodies may develop into cells very similar to 

 the mature egg cell, may be penetrated by spermatozoa, and may even be 

 fertilized and proceed a short distance in segmentation. There is perhaps 

 warrant for considering the polar bodies as rudimentary or abortive ova. 



The time of formation of the polar bodies varies in different animals. 

 In a few (echinoderms) they are formed before the sperm enters the egg. 

 In Ascaris they are both formed after the entrance of the sperm. In other 

 forms, like the mouse, the first polar body is formed while the egg is still in 

 the Graafian follicle, the second one after the entrance of the sperm. The 

 only recorded observations on maturation of the human ovum are those of 



