10 ‘ EDWIN G. CONKLIN 
(04) found in Dentalium; if it remains attached it fuses, at the 
close of the cleavage, with one of the cells, which then becomes 
larger than the other one. In this case the cleavage spindle 
is not eccentric and the furrow cuts down through the center of 
the egg until it reaches the yolk lobe when it turns to one side of 
the lobe leaving it attached to one of the cells. In this way a 
cleavage which began as an equal one becomes unequal. Where 
the spindle is eccentric from the start and the furrow does not 
pass through the center of the egg the yolk lobe is not prominent. 
In this way inequality of division may arise through the eccen- 
tricity of the spindle, or through the formation of a yolk lobe which 
remains connected with one of the two daughter cells, which 
would otherwise be equal. 
One cannot study the eggs of different animals without being 
much impressed with the fact that the distribution of yolk to the 
four macromeres is highly characteristic of different species and 
orders. Thus among prosobranchs the yolk is distributed either 
equally to all the macromeres, as in Crepidula, Fulgur, Trochus. 
etc., or if one of the macromeres is larger than the other three it 
is the left posterior macromere, D, as in Nassa, Urosalpinx, 
Tritia, ete. Among opisthobranchs, if the macromeres are 
unequal in size it is one or both of the anterior ones, A or Bb, 
which is the larger. Among pulmonates, so far as I recall, the 
macromeres are always equal in size. 
The fact that there are these characteristic differences in the 
sizes of the macromeres of different orders indicates that they 
have some characteristic cause; and the fact that in nearly re- 
lated species the macromeres may be equal or unequal indicates 
that in this case the cause is not a very general one. If one 
considers that the first and second cleavages normally pass 
through the egg axis, and that their position is determined by 
this structural feature, the unequal distribution of yolk to the 
four macromeres may be due to the localization of the yolk in 
different parts of the ovarian egg,—on the posterior side of the 
chief axis in prosobranchs, on the anterior side in opisthobranchs; 
while a larger or smaller yolk lobe would determine the degree 
of inequality of the macromeres in the different species. 
