MEIOSIS 17 



exchange of material takes place between them. As division of the nucleus 

 proceeds, separation of the conjugating chromosomes occurs, and the 

 two chromosomes of each pair pass to opposite poles of the spindle. It 

 will be seen, therefore, that in this division there has been no splitting of 

 the individual chromosomes as occurs in ordinary mitosis, but merely 

 a separation of two chromosomes which have come together temporarily 

 in syndesis. The number of chromosomes in the daughter nuclei are 

 thus half the original number. If the reduction division occurs at the 

 division of the primary spermatocyte, then the division of the nucleus 

 of the secondary spermatocyte is not a reducing one, the chromosomes 

 splitting longitudinally in the usual manner, so that the number is main- 

 tained. If the reduction occurs at the division of the secondary sperma- 

 tocyte, then the division of the nucleus of the primary spermatocyte is 

 of the ordinary type. In any case, the spermatids which become sper- 

 matozoa or gametes have half or the haploid number of chromosomes. 



In the case of the female cell similar changes occur. Cells called 

 oogonia grow into primary oocytes. A primary oocyte divides to give 

 rise to two cells, which are, however, unequal in size. The large one is 

 the secondary oocyte, and the small one the^^rs^ polar body. The secondary 

 oocyte divides into two cells, which are again unequal in size. One of 

 these is the ovum, and the other the second polar body. The first polar 

 body, which corresponds to a secondary oocyte, may itself divide into 

 two cells. The nuclear changes which occur in these divisions are similar 

 to those which occur during the divisions of the spermatocytes described 

 above, so that the number of chromosomes in the ovum is half the number 

 present in the o5gonia. There is this difference, however: Whereas each 

 primary spermatocyte gives rise to four spermatozoa, each primary oocyte 

 gives rise to one large ovum and two small polar bodies, or three if the 

 first polar body divides. By this arrangement the cytoplasmic part of 

 the ovum is increased at the expense of that of the polar bodies, which do 

 not proceed further to develop. Another difference between the ovum 

 and spermatozoon is that the centrosome of the ovum has disappeared, 

 though that of the latter has persisted. 



When conjugation or syngamy occurs, the nucleus of the microgamete 

 or spermatozoon unites with that of the macrogamete or ovum by a 

 process known as karyogamy to produce the nucleus or synkarion of the 

 zygote, which again has the diploid number of chromosomes arranged 

 in homologous pairs. The centrosome of the microgamete becomes the 

 centrosome of the zygote. 



The pairs of homologous chromosomes of the zygote can be recognized 

 through all the subsequent divisions of the cell down to the moment 

 when the new adult individual again produces spermatocytes or oocytes. 



I 2 



