40 EDWIN G. CONKLIN 
best determined by comparing subsequent stages with the 2-cell 
stage. Furthermore the nuclear volume in the 2-cell stage is 
less than at any other stage, and it consequently forms a good 
starting point for the study of nuclear growth. 
Finally, the volume of all the nuclei in the 70-cell stage, without 
attempting to determine the maximum volume of each nucleus, 
is shown in table 10. 
At the 70-cell stage the ectomeres are already closing over the 
yolk on the oral hemisphere, and it may be assumed that the 
cleavage will show no new tendencies as to the growth of nuclear 
substance until the embryo as a whole begins to grow. 
Whether nuclei are measured at either their maximum size, 
their minimum size or at a size intermediate between these two 
extremes, the rate of. growth during cleavage is found to fall far 
short of a doubling or increase of 100 per cent at each division. 
The average nuclear growth during early cleavage is not more 
than 5 to 9 per cent for each division, and in the later cleavage it 
falls as low as 1 per cent for each division. A growth of nuclear 
substance at this rate scarcely deserves to be designated as ‘phe- 
nomenal’ or ‘colossal.’ On the other hand, the protoplasm which 
is generally supposed to remain fixed in quantity during cleavage, 
increases at a more rapid rate than the nuclei, from the 1-cell 
to the 24-cell stages, as shown in table 8. In view of the facts 
here presented, even though it be for only a single species, the 
generally accepted conclusion as to the great increase of nuclear 
substance during cleavage, as contrasted with the lack of growth 
of the protoplasm, evidently needs revision, as do also the theories 
which have been founded upon this supposed fact. 
2. Nuclear growth during the cleavage of the egg of Fulgur. 
While my results are based largely upon the study of Crepidula 
plana they are not limited entirely to this species. The fol- 
lowing measurements of the nuclei of Fulgur carica are prob- 
ably not very accurate since they had to be made under a rela- 
tively low power objective (8 mm. apochromat) and since the 
material at my command did not permit the study of a large num- 
ber of eggs, and the selection of nuclei at maximum size. Never- 
