108 Cell Structure in Relation to Heredity 
may, like the chromosomes themselves, retain a definite position 
in the resting nucleus. Further, it may be assumed that during 
the separation of the chromosomes from one another and during 
their assumption of the rod-like form, the hereditary units become 
aggregated in the chromomeres and that these are characterised 
by a constant order of succession. The hereditary units then grow, 
divide into two and are uniformly distributed by the fission of the 
chromosomes between their longitudinal halves. 
As the contraction and rod-like separation of the chromosomes 
serve to insure the transmission of all hereditary units in the pro- 
ducts of division of a nucleus, so, on the other hand, the reticular 
distension of each chromosome in the so-called resting nucleus may 
effect a separation of the carriers of hereditary units from each 
other and facilitate the specific activity of each of them. 
In the stages preliminary to their division, the chromosomes 
become denser and take up a substance which increases their 
staining capacity; this is called chromatin. This substance collects 
in the chromomeres and may form the nutritive material for the 
carriers of hereditary units which we now believe to be enclosed in 
them. The chromatin cannot itself be the hereditary substance, as 
it afterwards leaves the chromosomes, and the amount of it is sub- 
ject to considerable variation in the nucleus, according to its stage 
of development. Conjointly with the materials which take part in 
the formation of the nuclear spindle and other processes in the 
cell, the chromatin accumulates in the resting nucleus to form the 
nucleoli. 
Naturally connected with the conclusion that the nuclei are 
the carriers of hereditary characters in the organism, is the question 
whether enucleate organisms can also exist. Phylogenetic considera- 
tions give an affirmative answer to this question. The differentia- 
tion into nucleus and cytoplasm represents a division of labour in 
the protoplast. A study of organisms which belong to the lowest 
class of the organic world teaches us how this was accomplished. 
Instead of well-defined nuclei, scattered granules have been described 
in the protoplasm of several of these organisms}, characterised by 
the same reactions as nuclear material, provided also with a nuclear 
network, but without a limiting membrane’. Thus the carriers 
of hereditary characters may originally have been distributed in 
the common protoplasm, afterwards coming together and eventually 
assuming a definite form as special organs of the cell. It may be also 
assumed that in the protoplasm and in the primitive types of nucleus, 
1 Bacteria, Cyanophyceae, Protozoa. 
2 This is the result of the work of R. Hertwig and of the most recently published 
investigations. 
