GROWTH AND REPRODUCTION 



763 



form. Further, when division takes place, the chromosomes, instead 

 of dividing lengthwise, divide transversely into halves, one half going 

 into each of the new daughter cells. In this way the daughter cells 

 have but half the number of chromosomes of the parent cell, and it 

 is assumed that the chromatin material may be of different quality 

 in the daughter cells. Thus, supppsing there are four chromosomes 

 (see Fig. 469), by this form of division various combinations may 

 occur, and it is suggested that it is in this way that hereditary characters 

 are handed on, and that the offspring may differ among themselves 

 according to the nature of the chromatin material derived from each 

 parent. 



In the case of the male gamete, the result of ordinary somatic 

 division yields cells known as spermatogonia. These develop into 

 cells kn^wn as primary spermatocytes. By meiotic division, the 

 primary spermatocytes give rise first to secondary spermatocytes, 

 and then to the spermatids, which develop into the functional sperma- 

 tozoa. We have thus (1) a somatic division stage of the primary 

 germ cell; (2) a growth period of the spermatogonia to spermatocytes; 

 (3) a maturation period of the spermatocytes to spermatids. 



PIG. 469. DIAGRAM TO SHOW THE DISTRIBUTION OF CHROMOSOMES TO THE DAUGHTER 

 CELLS IN THE MEIOTIC OR REDUCING FORM OF DIVISION. (C. E. Walker.) 



In the case of the female gamete, we have the corresponding 

 development by the somatic type of division of germ cell to oogonia. 

 Then follows the growth period, with the formation of the primary 

 oocytes from the oogonia. Lastly follows the maturation period, 

 with the meiotic division of the cells. The essential difference between 

 the maturation of the oocyte, as compared with the spermatocyte, is 

 that from the oocyte only one mature ovum, or egg, is formed, whereas 

 each primary spermatocyte yields four mature spermatids. In the 

 division of the primary oocyte, the secondary oocytes formed are the 

 large ovum and a first " polar body." The ovum again divides into 

 a mature egg and a small second polar body, while the first polar body 

 also divides. The result, therefore, of the maturation of the oocyte 

 is the formation of one functional mature egg and three functionless 

 polar bodies (Fig. 470). The result of the maturation of both sperma- 

 tocyte and oocyte is that the amount of chromatin material is reduced 

 to half, the full complement of chromatin being restored in the fusion 

 of the male and female gametes in the process of fertilization. 



