242 FINE-STRUCTURE OF PROTOPLASM It 



shape and agglomerate in a visible crystal lattice. The morphologicaE 

 analogy of chromonemata and virus, therefore, is no longer supported; the- 

 chemical comparison of both genes and viruses with enzymes is much more 

 convincing. 



Nucleus and cytoplasm. Considered from the morphological stand- 

 point, the secret of karyokinesis is evidently that the specific protein- 

 molecules, which serve as substratum to the genes, have to be carefully 

 transmitted to the daughter cells, preferably without any reciprocal! 

 changes of position along the chromatid. Their individuality and 

 specific spatial relationships were developed in the course of phylo- 

 genesis and the cytoplasm is not capable of re-creating them. The great 

 riddle of heredity therefore still is : How can a chromonema of such 

 complicated submicroscopic and amicroscopic morphology that it can 

 never be produced anew, bring forth its like from itself by longitudinal 

 division? This mysterious process must undoubtedly take place 

 frequently in the giant chromosomes of the Diptera, which are bundles 

 of similar chromonemata. It is as though the chromonemata served^ 

 as it were, as patterns for the creation of their like. It is known from 

 the evidence of the asymmetrical C synthesis (see p. 207) that certain 

 configurations are able to produce essentially the same morphological 

 forms in the amicroscopic region, but the refinements of this process, 

 and its mechanism are a mystery. For here, as contrasted with the 

 mode of action of the enzymes, it is not merely a question of fitting 

 a key to a lock, but of how the key produces one identical to it, or 

 the lock its exact like. 



If we take the specific structures to be a given fact, we come to an 

 important decision as to the morphological signification of the 

 nucleus. The gene-bearing protein threads are in a sense self-contained 

 and irretrievable structures and it therefore becomes clear why they 

 are not carried along by the cytoplasmic stream, but are localized at 

 a given spot. There they are withdrawn from the turbulent activity 

 of the cell and perform their directive and formative task as static 

 centres. 



It is evident from the heredity of cytoplasm (Wettstein, 1957) that 

 specific groups must also occur in it. These special structures, how- 

 ever, are not solitary, for parts of the cytoplasm are similar in their 

 behaviour to the whole cytoplast. Even fragments of the eggs of sea- 

 urchin without a nucleus can undergo a certain development involving 



