Abstract
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 producing.” 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 fertilized 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-determination 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 reduced 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 cytoplasm and subsequently degenerates without forming a spermatozoon.
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