GAMETOGENESIS 461 



1914) has traced the primordial germ-cells in the chick from a crescen- 

 tic area in the entoderm in front of the head end of the primitive streak, 

 through their migration to the gonad by way of the developing blood- 

 vessels. The earlier anatomists, on the other hand, derived the 

 primordial germ-cells from the mesothelial covering (germinal epi- 

 thelium) of the germ gland region of the primitive kidneys, the meso-- 

 nephroi. This mode of origin, either exclusively or in part, is still 

 supported by certain investigators. 



Whatever the actual source of origin whether entodermal or meso- 

 dermal, whether specific or otherwise and whenever the time of dif- 

 ferentiation, whether early or relatively late, the primordial germ-cells 

 in the gonad undergo extensive proliferation, increasing greatly in 

 numbers. The earlier generations may be inclusively designated pri- 

 mary spermatogonia or oogonia respectively, the final generation taking 

 the term secondary gonia. The question of the origin of the primordial 

 germ-cells bears upon the hypothesis of the continuity of germ-plasm. 



Spermatogenesis. Since gamete production is easier to follow, 

 though apparently more specialized, in the male, spermatogenesis is 

 advantageously first described. For the purpose we may employ the 

 active testis of Schist ocerca damnified, a common grasshopper. Grass- 

 hopper material of many species is peculiarly favorable for a demon- 

 stration of the process of spermatogenesis. This scheme will apply 

 with slight qualification to most grasshoppers. The reduced number 

 of chromosomes is the same, namely, 12 ; moreover, this material shows 

 peculiarly well the accessory or sex chromosome, and thus serves as 

 a splendid basis for a discussion and comprehension of the salient facts 

 and recent theories touching the problems of the determination, control, 

 and inheritance of sex. 



The grasshopper testis is a long tubular structure subdivided into 

 a number of compartments, or cysts, each filled with cells of ap- 

 proximately the same stage of development, the successive stages in 

 spermatogenesis being represented in successive cysts from distal to 

 proximal pole. For purpose of readiest study there is required, then, a 

 median longitudinal section of ah entire testis in active condition. As 

 is obvious, such material is useful also for illustrating mitosis but, with 

 the exception of the spermatogonial and second maturation mitoses, 

 cannot be regarded as typical; but for application and extension of a 

 preliminary knowledge of simple mitosis it is perhaps unexcelled. It 

 must be emphasized that this peculiar type of cell division, charac- 

 terized chiefly by the phenomena of synapsis and tetrad formation, in- 



