INTRODUCTION 13 



the primordial germ-cells, known as ovogonia and spermatogonia 

 are very similar in their morphological characters; both kinds 

 are small, yolkless cells containing the typical or somatic number 

 of chromosomes; they multiply rapidly by karyokinetic division. 



At the end of this period multiplication ceases and the germ- 

 cells increase in size (period of growth). They are now known 

 as ovocytes and spermatocytes of the first generation. The 

 growth of the ovocyte is much greater than that of the sperma- 

 tocyte; deposition of yolk occurs in the ovocyte during this 

 period; and in some animals a small quantity of yolk-granules 

 may be found even in the spermatocytes. Another character- 

 teristic feature of the period of growth is the reduction of the 

 number of chromosomes to one half of the typical number, which 

 takes place, according to the current conception, by union of the 

 chromosomes in pairs (synapsis) forming one half of the somatic 

 number of chromosomes, which are, however, bivalent and are 

 known as tetrads. 



At the end of the period of growth the ovocyte of the first 

 generation is usually many times larger than the spermatocyte, 

 owing mainly to the amount of yolk formed. But the two kinds 

 of cells are precisely alike in nuclear constitution. Then comes 

 the period of maturation, which is the same in both kinds of cells 

 with reference to the nuclear phenomena, but very different as 

 regards the behavior of the cell-body. The maturation consists 

 of two rapidly succeeding karyokinetic divisions: in the case of 

 the spermatocyte the first division results in the formation of 

 two similar cells, the spermatocytes of the second order, and the 

 second maturation division divides each of these equally, forming 

 two similar spermatids, so that four equal and similar spermatids 

 arise from each spermatocyte of the first order. Each spermatid 

 then differentiates into a single spermatozoon. In the case of 

 the ovocyte of the first order, the first maturation division is 

 exceedingly unequal; the smaller cell is known as the first polar 

 body, but both cells are ovocytes of the second order. The second 

 maturation division usually involves only the large secondary 

 ovocyte; it is as unequal as the first division and results in the 

 formation of a second polar body. The division of the first polar 

 body, where it occurs, is equal. Thus the net result of the matu- 

 ration division of the ovum is the production of three cells (four 

 if the first polar body divides), viz., the two (or three) polar bodies 



