TEE CELLULAR BASIS OF HEREDITY 123 



3. The Origin of the Sex Cells 

 The sex cells are the latest of all cells of a developing organism to 

 reach maturity, and yet they may be among the earliest to make their 

 appearance. Every sex cell, like every other type of cell, is a lineal 

 descendant of the fertilized egg (Fig. 22), but the period at which the 

 sex cells become visibly different from other cells varies from the first 

 cleavage of the egg in some species to a relatively advanced stage of 

 development in others. 



(a). The Division Period. Oogonia and Spermatogonia 



"When the primitive sex cells are first distinguishable they differ 

 from other cells only in the fact that they are less differentiated; they 

 have relatively larger nuclei and smaller cell bodies — a condition which 

 is indicative of little differentiation of the cell body since the products 

 of differentiation such as fibres, secretions, etc., swell the size of the 

 cell body, but do not contribute to the growth of the nucleus. These 

 primitive sex cells or gonia divide repeatedly, but the oogonia grow 

 more rapidly and divide less frequently than the spermatogonia. As a 

 result of this difference in the rate of growth and division the sperma- 

 togonia become much smaller and immensely more numerous than the 

 oogonia. This period in the genesis of the sex cells is known as the 

 division period (Fig. 22). 



(&). The Growth Period. Oocytes and Spermatocytes 



This period of rapid cell divisions is followed by a period of growth 

 without division during which the developing sex cells are called 

 primary oocytes or spermatocytes. This growth period may be very 

 long in the case of the oocytes, lasting, for example, in the human 

 female from the time of birth to the end of the reproductive period; 

 during this long time the oocytes in the ovary probably never divide — 

 there are as many of them at birth as at any later time; during this 

 period of growth the ovarian egg becomes relatively large, in some 

 animals, e. g., birds, the largest of all cells. The growth period of a 

 spermatocyte lasts for a briefer time than does that of an oocyte so that 

 the former remains relatively small (Fig. 22). 



All of the cell divisions which take place during the division period 

 are of the usual kind, in which every chromosome splits lengthwise into 

 two and the two halves then separate and move to opposite poles of the 

 spindle where they break up into threads and granules and form the 

 daughter nuclei, as is shown in Fig. 24. But during the growth period 

 of the oocytes and spermatocytes the chromosomes form a closely wound 

 coil of long chromatin threads (Fig. 33), and when these threads uncoil 

 later it is seen that the chromosomes have united in pairs (Figs. 33 D 

 and E, 34 B, 35 B) ; this process is known as synapsis, or the conjuga- 



