2O6 T. H. MORGAN. 



development of the relatively enormous reproductive organs of 

 the male. The testis is present as a large mass of cells at the 

 time when the blastoderm is first laid down, and the spermato- 

 gonial divisions occur at this time. The two spermatocyte divi- 

 sions occur when the embryonic plate is forming, and before the 

 fibers appear in the nervous system. The size of the mass of 

 spermatocyte cells is very large compared to the rest of the 

 embryo, and it fills the greater part of one half of the egg. The 

 spermatozoa are fully formed at the time when the embryo is 

 developed. 



The precocious development of the large testicular mass in the 

 male egg suggests that a preexisting mass of cytoplasm from 

 which the testis develops may be present in the egg. The cen- 

 tral mass of cytoplasm might supply such material, but I have not 

 been able to make out any connection between the two. The 

 result is due rather to the more rapid development of certain em- 

 bryonic cells than to the presence of any large cytoplasmic mass 

 of preformed material. The question arises whether we are to 

 regard the large eggs that produce the females as doing so sim- 

 ply because they are large, or are they large because they are 

 already female eggs ? The latter alternative seems the more 

 probable. In the light of certain recent experimental results, 

 more especially those of Driesch and of Godlewski it seems 

 highly probable that the early development of the embryo is due 

 almost entirely to cytoplasmic influences. If this is true also in 

 the case of the eggs of Phylloxera, then I think we may safely 

 ascribe the difference in the size of the male and female eggs to 

 the difference in the kinds of cytoplasm that are present when the 

 egg is fully formed, so that the immediate determination of the 

 sex is a cytoplasmic phenomenon. Whether this cytoplasmic 

 difference can be traced to a preexisting cytoplasmic basis, or to 

 nuclear influence, or to the influence of external conditions is 

 quite unknown, but in the absence of any nuclear differences it 

 seem questionable whether we should assume that such exists. 



