274 — T. H. Morgan 
producing” sperm. Here we meet with one of the many paradoxes 
to which the view, that the sex chromosomes are sexually different, 
leads. 
On the contrary assumption that the male egg eliminates the 
male chromosomes we leave unexplained the essential point of 
what then determines that that egg is to become a male. Other 
combinations of the sex chromosomes are conceivable, but all lead 
alike to contradiction or difficulties on the assumption that the 
chromosomes are really differentials and that their elimination 
from the egg and sperm is discriminative. 
The opposite point of view looks upon the two pairs of chro- 
mosomes of the female and the single pair remaining in the male 
egg as strictly equivalent, pair for pair. As long as either member 
of a pair is eliminated from the male egg the essential conditions 
for male production are fulfilled. The union of the homologous 
members of the pair in Phylloxera caryzcaulis would mean that 
this is a step preparatory to their separation at the next division. 
The pairs might be turned either way on the spindle since it would 
be a matter of indifference which member of the pair was elimi- 
nated and which remained in the egg. The separation would be 
fortuitous. ‘The difference between male and sexual female would 
be a quantitative difference so far as the chromosomes are con- 
cerned. But since the total number of chromosomes is the same 
in the parthenogenetic and the sexual females some other condi- 
tion must determine the transition from the one to the other. 
What that other condition could be unless synapsis we do not 
know any more than why in the body the cells are differentiated 
in many directions without any chromosomal differences to dis- 
tinguish them. 
If the transition from a parthenogenetic female to a sexual 
female can occur without chromosomal diminution—and the transi- 
tion involves as vast a number of differences as those that distin- 
guish male from female—may not the same sort of change take 
place in the case of the transition from the parthenogenetic to the 
male form? We have seen, in fact, that a change has taken place 
a generation before the male egg appears that is prophetic of the 
change that comes later. May not this change be the real condi- 
