SEXUAL CELLS. 



269 



cells). Later they are scattered about in the mesothelium of the abdominal 

 cavity and finally they migrate into the genital ridges to become spermatogonia 

 in the male, or the corresponding oogonia in the female. Similar cells have 

 been found in reptiles and in the older mammalian embryos. The early segre- 

 gation of the sex cells has been cited in favor of the opinion that acquired characters 

 cannot be transmitted; the cells have been considered as quite independent 

 of the body in which they are lodged, which serves as their "trustee." It has 

 not been established, however, that these cells in mammals are earlier or more 

 completely separated from the rest of the body than are those of other organs. 



Although in consideration of the variety of cells to which it may give rise 

 the fertilized ovum is classed as the least differentiated of cells, yet the sexual 

 cells which unite to produce it are highly differentiated both in form and function. 



Spermatids. 

 Sustentacular cell. 

 Spermatogonium. 



Blood vessel with 

 blood corpus- 

 cles. 



Spermatids. ' \ 1 



Sustentacular cell. Spermatogonia, beneath Sustentacular cells. 

 large spermatocytes. 



FIG. 305. CROSS SECTIONS OP SEMINIFEROUS (CONVOLUTED) TUBULES OF A MOUSE. X 360. 



Neither of them normally has the power for further mitosis, yet when combined 

 they produce a cell in which this capacity is unsurpassed. The rate of cell 

 division falls as the embryo grows, and is restored only in the sexual cells differ- 

 entiated for this purpose. 



The development of the mature sexual cells in the male, the sper- 

 matozoa, occurs in the convoluted tubules of the testis, beginning at puberty 

 and continuing throughout life. With advancing age the rate diminishes. 

 Since about 60,000 spermatozoa occur in a cubic millimeter of seminal 

 fluid, it has been estimated that 340 billions are produced in a lifetime. 



