490 T. H. MORGAN is 
in the egg. The female-producing sperm brings in during ferti- 
lization a large X and a small x which brings the number back 
to the six chromosomes (in reality eight since two are double, 
X and x) of the stem-mother’s soma. 
THEORETICAL INTERPRETATION OF THE RESULTS 
From the results given in preceding sections I draw the follow- 
ing conclusions. The cells of the stem-mother (that comes from 
the fertilized egg) contain six equal or nearly equal chromosomes. 
Two of these, that I call X, have attached to them two smaller 
chromosomes that I call small «. The stem-mother’s cells have 
therefore in reality eight chromosomes. ‘The eggs produced 
by the stem-mother also contain these six (or eight) chromosomes 
that appear in the equatorial plate of the polar body. One polar 
body is extruded. The division of the chromosomes has not 
been observed. Iassume that at this time all of the chromosomes 
divide equally, except in the case of those eggs that will become 
male-egg producers. In these eggs one of the small 2’s passes 
undivided into the polar body. Prestmably it passes out at- 
tached to the outgoing half of the larger _X, with which it has been 
fused. Unless it separated from the large X it might not 
appear as a lagging chromosome at the time; if it became detached 
it might appear as a lagging chromosome; or both the large out- 
going half of the large X with its attached small « might lag behind 
the rest. Further work will be necessary to settle this point. 
This kind of egg, after the polar body is extruded, will contain 
six chromosomes, one having been reduced in size by the loss of 
the small #. This group appears in the equatorial plate of the 
polar spindle of the small ‘male’ egg. The difference in the size 
relations of the chromosomes observed in the polar spindle of 
this egg, as compared with the size relations of the chromosomes 
in the stem-mother’s egg is accounted for by the loss of one chro- 
mosome—the small x. If eight chromosomes are present in the 
stem-mother’s cells and eggs there are only seven in the body 
cells and in the eggs of the male-producers. When the polar 
body of the male-producing egg is formed all the chromosomes 
divide except one, which, lagging on the spindle, finally passes 
