CHAPTER XXIII 



ORIGIN OF THE SEX CELLS 



The first step in the production of a new individual sexually is the 

 formation of sex cells. This takes place in gonads which arise from 

 the germinal epithelium, which in turn is developed from the cells lining 

 the coelom, or body cavity. 



133. Gametogenesis. — The origin and development of the sex cells 

 are termed gametogenesis. This may be divided into spermatogenesis, 

 which deals with the male germ cell, called the sperm, sperm cell, or 

 spermatozoon; and oogenesis, which deals with the female germ cell, 

 called the egg cell, or ovum. 



In all references to the male germ cell in this text it will be called a 

 sperm cell. The objection to the word spermatozoon is that it perpe- 

 tuates an error; it means, literally, "sperm animal" and was proposed at 

 a time when it was believed that these cells were themselves animals 

 living in the bodies of higher animals. The term egg cell is preferred 

 to ovum because the latter has been used both in this sense and also to 

 apply to the whole egg. 



Both processes, spermatogenesis and oogenesis, begin (Figs. 41 and 

 42) quite early in the life of the embryo by the setting aside of a pri- 

 mordial germ cell from which come all of the sex cells which will be devel- 

 oped in that animal's body. This cell multiplies by repeated divisions 

 until a very large number of cells is produced; the time during which 

 this occurs is called the ynulti plication period. In spermatogenesis these 

 cells are known as spermatogonia; and in oogenesis, oogonia. When the 

 animal becomes sexually mature, these cells undergo the processes of 

 growth and maturation, the growth period involving both an increase in 

 the size of the cell and a union of hke chromosomes in pairs. This union 

 of chromosomes is termed synapsis. At the end of the growth period 

 the male cells are termed primary spermatocytes; and the female cells, 

 primary oocytes. From this time on the processes of spermatogenesis 

 and oogenesis differ. 



134. Spermatogenesis. — The periods of multiplication and growth 

 having been completed in spermatogenesis, the maturation period fol- 

 lows (Fig. 41). The primary spermatocyte undergoes two maturation 

 divisions. The first results in the formation of two secondary spermato- 

 cytes, and the second in the formation of two spermatids from each of these 

 secondary spermatocytes, making four spermatids altogether. The 



107 



