CELL SIZE AND NUCLEAR SIZE 51 
the same generation, though naturally it is more difficult to detect 
size differences in objects as small as chromosomes than in entire 
nuclei. Where the differences in nuclear volumes are great one 
can always detect corresponding differences in chromosome vol- 
umes. 
The chromosomes of the spermatid are usually smaller than 
those of the o6dtid, but when the chromosomes of the first cleavage 
spindle appear, those from the sperm nucleus are usually as large 
as those from the egg. The reason for this is to be found in the 
fact that both grow, after fertilization, in the same medium, the 
egg plasma, and for approximately the same length of time. 
e. Plasmasomes. The conclusion that large nuclei have large 
chromosomes, and vice versa, also applies to the sizes of nucleoli 
(plasmasomes) ; they are larger in large nuclei than in small ones. 
However in this case another factor is involved for the size of 
nucleoli is not only dependent upon the size of the nucleus, but 
also upon the length of the resting period; indeed the latter seems 
to be the more important factor of the two. The largest of all 
nucleoli is the one found in the germinal vesicle, at the close of the 
longest resting period in the entire life cycle. In these gasteropod 
eggs the next largest nucleoli are found in the cells 4A—4D and 
4a—4¢ (fig. 6) in which the resting stage is particularly long. The 
nuclei of the cells 4A—4D are of the same size as those of 2A—2D 
viz. 184 in diameter, but the nucleoli of the former have about 
three times the diameter of those of the latter. 
In earlier stages of cleavage where the blastomeres are dividing 
rapidly it is difficult to compare the sizes of nucleoli, not only 
because their number varies considerably, but also because each 
plasmasome is usually surrounded by a layer of chromatin gran- 
ules which renders exact measurements difficult. The number of 
plasmasomes appears to depend to a large extent upon the degree 
of fusion of an originally large number of separate plasmasomes. 
When chromosomes are isolated so that each gives rise to a dis- 
tinct vesicle, each may contain a minute plasmasome, and there 
may be as many of these as there are chromosomal vesicles. In 
Crepidula the number is always greatest during the earlier stages 
of the resting period; during the later stages they appear to fuse 
