76 EDWIN G. CONKLIN 
the vegetal pole, the normal size relations of the germ nuclei is 
reversed, the sperm nucleus becoming larger than the egg nucleus 
as shown in fig. 11. Godlewski (’08) holds that the size of the 
sperm nucleus depends upon the time which elapses before its 
union with the egg nucleus; it also depends, as I have shown, 
upon the quantity of cytoplasm in which it lies. We conclude 
therefore, that in all animals the relative sizes of egg and sperm 
nuclei are dependent upon the amount of cytoplasm in which 
they lie, and upon the length of the growth period (interkinesis). 
In this connection it may be worth while to remark that one 
reason why the rhythm of cleavage, in Boveri's, Driesch’s, and 
Godlewski’s experiments, follows the maternal rather than the 
paternal type may be found in the fact that the rate of growth 
of the nucleus is dependent upon the quantity and quality of the 
protoplasm of the egg. 
In the cleavage of the egg the size of the nucleus is dependent 
upon the quantity of protoplasm in which it lies, as shown by figs. 
12 to 20. In eggs subjected to strong centrifugal force the egg 
contents separate into three zones, a yellow zone of yolk at the 
distal (heavy) pole, a gray zone of oily and watery substance at 
the central (light) pole, and a clear zone of protoplasm between 
these two. It is the latter substance which contributes to the 
growth of the nucleus, as is shown by such cases as fig. 16 in which 
the gray substance was centrifuged out of the egg and practically 
all of the yolk thrown into one of the blastomeres, and most of 
the clear protoplasm into the other; the nucleus in the blastomere 
which contains yolk but little or no protoplasm has scarcely 
grown at all, the one in the cell containing the clear protoplasm, 
but without the gray substance, has grown enormously. Sim- 
ilar, though less striking, differences in the sizes of nuclei, depend- 
ing upon the quantity of clear protoplasm in the cell, are found 
in all the eggs figured. In centrifuged eggs the nucleus always 
occupies the middle zone, and as I have just shown it grows at the 
expense of substance received directly from this zone. The fact 
that the specific gravity of the nucleus and of this middle zone are 
the same, is probably due to the fact that so much of the absorbed 
nuclear material is from this zone. 
