INTRODUCTION 15 
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 ovoecyte 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 
