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 1 , characterised by 

 the same reactions as nuclear material, provided also with a nuclear 

 network, but without a limiting membrane 2 . 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. 



