INTRODUCTION 



11 



unaltered nuclear content that the germ cells have their origin. 

 But even in this instance the separation of germ cells from somatic 

 cells is not so direct as this narration might imply. In each 

 of about the first five divisions of the cell with its full complement 

 of chromatin one of the two resulting cells undergoes a chromatin 

 diminution but at the end of the fifth or sixth cleavage the germ 

 plasm has been isolated and in all subsequent divisions every 

 blastomere gives rise to either 

 somatic or germ cells. 



In several other instances, it 

 has been noted that the cells 

 which later produce the gametes 

 are distinguishable early in em- 

 bryonic life. In insect embryos, 

 the cells from which the gametes 

 are formed are distinctly larger 

 than the somatic cells. Thus 

 in the fly Miastor, the primor- 

 dial germ cells (Fig. 8 gc) are 

 readily distinguishable from the 

 somatic cells (cl) early in the 

 cleavage of the egg. 



Gametogenesis and Fertiliza- 

 tion. — Even though the cells 

 which later go to form the 

 gametes are early distinguish- 

 able from the somatic cells, they 



must pass through a COmpli- Fig. 8.— Development of a centro- 

 Cated series of changes before l e c i t h a l egg of the fly Miastor. 

 . , , , ^ . . Blastula stage showing germ cells (gc) 



they are capable of union m 

 fertilization. These changes are 

 collectively termed gametogene- 

 sis or, in the male, spermato- 

 genesis and in the female, oogenesis (Fig. 9 B-D). Three 

 periods are recognizable in gametogenesis: a multiplication 

 period wherein the relatively small number of primordial germ 

 cells is greatly augmented; a growth period, which involves fun- 

 damental changes in the nuclear organization of the cell and 

 in relative size; and finally a maturation period during which the 

 chromosomes in the gametes are reduced to one-half the number 

 characteristic of the somatic nuclei. The modified nuclear divi- 



at posterior extremity readily distin- 

 guishable from the other blastomeres 

 {cl) . {From Shull, LaRue, and Ruthven 

 after Hegner). 



