492 T. H. MORGAN 
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out bodily into the polar body dividing at this time as do 
also the members of the outer group of chromosomes. The 
result is that five visible chromosomes remain in the male egg, 
in reality six chromosomes, since the large X and the small x 
are attached to each other. In the body cells of the male these 
two X’s often remain united, but sometimes partially separate. 
When reduction takes place three visible chromosomes are pres- 
ent in the spermatocytes (one of these three is the fused pair). 
Two of these divide equally at the first division, and one (the 
fused pair) lags behind and finally passes to the female-producing 
sperm. ‘Toward the end of this division the large X and the 
small x, not infrequently partially or even completely separate. 
In the second spermatocyte division all the chromosomes of 
the female-producing cell divide equally, giving rise to two func- 
tional sperm containing three visible chromosomes, or four in 
reality since one is double. The class of cells without the X’s 
degenerate. 
Returning again to the stem-mother in order to trace the history 
of the female line I assume that when the polar spindle divides 
all of the eight chromosomes divide, leaving eight in the egg (six 
visible) of which there are two pairs, each containing a large X 
and its attached small x. The eggs develop into the female 
producers, whose polar spindles contain six equal chromosomes 
like those of the polar spindle of the stem-mother. The larger 
number of chromosomes in the female-producing egg accounts 
for the larger size of the egg, as compared with the male-pro- 
ducing egg, which, as shown above, contains one less chromosome 
(the small x). When the polar body is set free the chromosomes _ 
divide equally, six (in reality eight) passing out, and six (or eight) 
remaining in the egg. The body cells of the sexual female con- 
tain therefore six (eight) chromosomes. <A reduction division 
occurs in the sexual eggs, so that three (in reality four) chromo- 
somes appear. When the polar bodies are formed—there are 
two of them to judge by analogy with the aphids—three whole 
chromosomes (in reality four) are given off at one of the divisions 
so that three (in reality four) remain in the egg. The female- 
producing spermatozoon introduces into this egg during fertili- 
