Structural Diferentiation of the Nucleus 113 



cytologists, together with cytogeneticists who seek data from irradi- 

 ation experiments as well as direct observation, fall into two schools 

 regarding the time at which chromosome splitting takes place. And 

 the submicroscopic data that fit in with the views of one school 

 regarding meiosis seem to me to be against that school regarding 

 mitosis. Darlington (1937) and his school consider that meiotic 

 chromosomes reproduce themselves during late pachytene. This 

 fits Caspersson's data on the greatest concentration of nucleic acid 

 occurring just before the tetrad split becomes visible in grasshopper 

 spermatocytes. But Darlington is likewise convinced that repro- 

 duction of somatic chromosomes occurs during the resting stage 

 immediately preceding the mitosis in which separation of chromo- 

 some halves occurs — his whole "precocity theory" relating meiosis 

 to mitosis stands or falls on this diff'erence in the time of splitting 

 of meiotic and mitotic chromosomes. And during the resting stage, 

 when the chromosomes are believed by this school to split, the 

 nucleic acid concentration of the nucleus is at a minimum! One 

 should add that Darlington and La Cour (1940) state that "the 

 chromosome cycle is adjusted to have a maximum aggregate nucleic 

 acid attachment at metaphase" and that "during the resting stage 

 it is present in the nucleus in smaller quantity and largely unattached 

 to the chromosomes," but the basis for this opinion is not entirely 

 clear. However this may be, the other school, to which I belong, 

 is convinced that chromonemata (and genes) reduplicate themselves 

 at least one and probably two or more cell generations before the 

 mitosis in which the "split" becomes "effective." It is clear from the 

 work of Caspersson and Schultz that nucleic acid production is con- 

 trolled by genes, but to me the converse conclusion, that nucleic acid 

 is essential to gene reproduction, is not fully established by present 

 data, and it seems most unlikely that the obvious, wide separation of 

 chromosome halves that is seen in the earliest prophase of mitosis 

 is the chronologically immediate result of gene reproduction. Rather, 

 it is related to the onset of mitosis; and the occurrence of polytene 

 chromosomes, apart from all other evidence, shows per contra that 

 special physiological conditions not necessarily correlated with gene 

 or chromonema reproduction are involved in the initiation of nuclear 

 division. 



Before proceeding to other aspects of Caspersson and Schultz's 

 work we must recall the differentiation of the chromosomes into 

 regions which Heitz (1935) termed "euchromatic" and "hetero- 

 chromatic." Here we must consider data from all four levels. Long 



