CELL SIZE AND NUCLEAR SIZE til 
Apart from the relation of the yolk lobe to unequal cleavage 
Wilson (’04) has shown that it bears some relation to the forma- 
tion of the pretrochal region in the larva of Dentalium; when the 
lobe was removed the pretrochal organs failed to develop. What 
the morphogenetic factors are, which are located in the yolk lobe, 
is not known, but the significance of the lobe can scarcely be for 
the formation of the pretrochal region, since in animals with no 
lobe or with a very minute one these regions form quite as well 
as in those with a large lobe. 
These explanations refer to the ‘‘prospective significance” of 
the yolk lobe, and I know of no certain evidence as to the cause 
of its formation. The fact that such a lobe is present in almost 
all gasteropod eggs, differing only in size in different species, and 
that it is present in the eggs of annelids and a large number of 
other animals, indicates that it has some cause of general occur- 
rence. In 1897 I suggested that the yolk lobe marks the point 
of attachment of the ovarian egg to the follicular wall. At this 
point there is left a little mass of protoplasm on the surface of 
the egg, and here there is a weak spot in the protoplasmic pellicle 
which surrounds the egg. If the egg is put under pressure the 
yolk may be caused to flow out at this point, and in the increased 
tension which accompanies mitosis a yolk lobe is often pushed 
out at this spot. 
On the whole then, it seems probable that the yolk lobe rep- 
resents a temporary extrusion of egg substance during mitotic 
pressure at the former point of attachment to the ovarian wall, 
and that as a result of the presence of a large lobe of this kind, 
the first and second cleavages may be rendered unequal though 
the intersection of the furrows may lie in the egg axis and in the 
polar diameter of the egg. 
In this connection one recalls the ‘ Dotterball’ and the ‘Granula- 
ball’ observed by Hogue (710) and Boveri (10) in centrifuged 
eggs of Ascaris. Boveri comes to the conclusion that these are 
formed because they lie outside the influence of the asters or 
spheres: ‘‘Man k6énnte vielleicht sagen:—der von einen Sphire 
eingenommene Plasmabezirk sucht sich von allem was ausserhalb 
dieses Wirkungskreises liegt, abzuschniiren,” (p. 123). He sup- 
