170 CELL DIVISION : DEVELOPMENT OF GERM-CELLS 



ing nucleus again retires into the interior of the egg-cytoplasm 

 to await the advent of the spermatozoon. The egg is now 

 mature, and capable of fertilisation. It contains half the 

 number of chromosomes that were in the oogonium. 



Examine sections of the ovary of A scar is with the high power 

 in order to see the formation of the polar bodies. Sections of the 

 ovary of the frog, rabbit, or mouse also show the growth of t/te 

 oocyte. 



B. Spermogenesis. 



The primary sperm - cells can be recognised at an early 

 stage of development, in this respect agreeing with the 

 primary ova. These spermogonia, as they are called, 

 become imbedded in the peritoneum on either side of the 

 aorta. Over this area the peritoneum develops a series of 

 tubular folds which constitute the testis. In this position the 

 spermogonia divide either by the direct method, or more 

 generally as described for a tissue-cell on p. 165. The full 

 number of chromosomes is present. After a considerable 

 number of such divisions the resulting cells eease to multiply 

 and commence to grow. They may now be called primary 

 spermocytes. Each spermocyte is capable of giving rise to four 

 spermatozoa. The first step in the process is the division of a 

 spermocyte into two cells. This division always takes place 

 by karyokinesis, and is accompanied by a reduction in the 

 number of chromosomes to half the initial number (Fig. 40). 

 A nuclear figure is formed, but as the chromatin breaks up 

 into chromosomes these bodies fuse in pairs to form double 

 chromosomes. Each spennocyto behaves like an oocyte, two 

 equal secondary spermocytes are formed, and these rapidly 

 dividing again give rise to four speriuatids, each of which has 

 half the number of chromosomes present in the primary 

 spermocyte. These four spermatids correspond with the ripe 

 ovum and its three polar bodies. 



The spermatids are, however, unripe spermatozoa. To 

 become mature they have to go through a complicated develop- 

 ment. Each of these cells contains a centrosome and a nucleus 

 imbedded in a spherical mass of cytoplasm. The first change 

 is the outgrowth of a delicate thread from the centrosome. 



