I GROWTH OF THE GERM-CELLS 23 



development, for high correlation probably points to depen- 

 dence in development of one part upon another, a factor we 

 shall see to be of the greatest importance, and what throws 

 light upon one may help to explain the other. 



The same may be said of the change in variability, for 

 a knowledge of what brings about this change is a step 

 towards the knowledge of the causes of the phenomenon. 



We turn next to the second process in development, the 

 division of the ovum, preceded by the karyokinetic division of 

 the nucleus. A discussion of the problems presented by these 

 phenomena requires, however, a brief preliminary account of 

 the structure of the germ-cells, and of their union in the act 

 of fertilization. 



The germ-cells the ovum and the spermatozoon though 

 very dissimilar in structure, except in one respect, are the 

 products of a history which is almost identical in the two 

 sexes. In this history there are three periods, of multiplica- 

 tion, of rest and growth, and of maturation. In the first 

 period the young germ-cells derived from the primordial germ- 

 cells divide many times, showing at each division of the nucleus 

 the same number of chromosomes as is seen in the tissue-cells. 

 This number, since it is usually even, we shall speak of as 2n. 

 These cells are spoken of as oogonia and spermogonia respec- 

 tively. 



After a while the divisions come to an end and each cell 

 grows into a primary oocyte or a primary spermocyte as the 

 case may be. It is at this point that the difference between 

 the two sexes becomes manifest. 



In the male the growth is not great, the nucleus passes 

 through the prophases of the first maturation division lep- 

 totene, synaptene, pachytene, and formation of heterotypic 

 chromosomes and then immediately proceeds to the two actual 

 divisions of the third or maturation period. As is well known, 

 the number of chromosomes is reduced from the somatic 

 number (2n) to the germ number (n), and very possibly this 

 occurs, as many hold, by a separation of the dissimilar halves 

 of the n bivalent heterotypic chromosomes in the first division, 



