CELL SIZE AND NUCLEAR SIZE 69 
plasm is essential to the growth of the nucleus and of the chroma- 
tin; on the other hand chromatin is essential to the growth of 
cytoplasm, or to the conversion of yolk or food substances into 
cytoplasm. The life of the cell consists in an interchange of 
materials between the nucleus and the cytoplasm; the one cannot 
grow in the absence of the other. This conclusion agrees with 
the generalization of Godlewski (10): ‘“‘Zuerst das Bildingsmate- 
rial geliefert und von den betreffenden Regeneratskomponenten 
zum Protoplasm assimiliert wird, dass dagegen in der zweiten 
Regenerationsphase dieses Protoplasma sich wenigstens teilweise 
zur Kernsubstanz transformiert”’ (p. 88). 
The question has been much discussed as to whether the nuclei, 
and more particularly the chromosomes of the germ cells, are 
the sole ‘bearers of heredity,’ as Weismann, and many others, 
have maintained. We have experimental evidence that the 
cytoplasm cannot form chromatin in the absence of preéxistent 
chromatin. On the other hand there is no certain evidence that 
the chromatin can form cytoplasm in the absence of preéxisting 
cytoplasm. The experiment described above is not entirely 
conclusive, for while chromosomes in a yolk field form cytoplasm, 
it is probable that a minimal amount of cytoplasm is-left in the 
yolk field, and it may be said that this merely grows by assimila- 
tion of yolk. On the other hand my experiments show that 
where we have equal division of the chromosomes and unequal 
division of the protoplasm we may have regulation and normal 
development; whereas this never follows abnormal distribution 
of the chromosomes; in other words protoplasmic abnormalities 
are capable of regulation when the nucleus is normal, but the re- 
verse is not the case. The nucleus is the regulating center of the 
cell, and it is probably also the assimilating center. And since 
both of these functions are involved in inheritance, to this extent 
at least the nucleus may be said to be the inheritance center. 
Fig. 17 represents an egg which was centrifuged for four hours 
during the first cleavage and was then placed under normal con- 
ditions for six hours before being killed. The polar body marks 
the original animal pole and in the centrifuging most of the yolk 
was thrown to this pole, most of the cytoplasm to the opposite 
