56 



Scientific Proceedings (27). 



30 (286) 



The production of two kinds of spermatozoa in phylloxerans 

 — functional " female producing " and 

 rudimentary spermatozoa. 



By T. H. MORGAN. 



\_From the Laboratory of Experimental Zoology, Columbia 

 University.'] 



The work of McClung, Stevens, and Wilson has shown in 

 the group of insects that sex-determination is associated with the 

 presence of two kinds of spermatozoa — "male and female pro- 

 ducing." From this point of view sex is determined by the 

 sperm and not by the eggs in those species of insects in which 

 parthenogenesis does not occur. Within the group of insects 

 there are other species in which parthenogenesis appears as a part 

 of the regular life-cycle. Such cycles are shown especially in 

 the groups of aphids and phylloxerans. In these, all of the ferti- 

 lized eggs produce females only, while from the parthenogenetic 

 eggs both males and females develop. Hence it is evident that, 

 in these groups at least, the egg may be sex-determining, but how 

 this could take place has not been discovered. 



In several species of Phylloxera that I have studied some 

 facts have come to light that go far towards explaining sex-deter- 

 mination in this group. I shall describe first the spermatogenesis 

 of a species that contains so small a number of chromosomes 

 that the number can be counted accurately, not only in the re- 

 duced number of the spermatogenesis but in the somatic cells as 

 well. 



The reduced number of chromosomes is three. In the first 

 spermatocyte division two of these divide equally, but the third 

 lags behind the others, and finally in the very last stages of this 

 division it retreats to one of the poles. Thus there are three 

 chromosomes in one of the two daughter cells, and only two in 

 the other. Still more significant is the fact that the cell with the 

 fewer chromosomes is very small ; it contains very little cyto- 

 plasm and subsequently degenerates without forming a spermato- 

 zoon. In the second spermatocyte division all three chromo- 



