Structural Differentiation of the Nucleus 119 



deeply with Feulgen during the "resting" stage, they must, however, 

 be rich in desoxyrihonucleic acids at that time. The work of McClin- 

 tock (1934) on the nucleolar-forming body in Maize, and that of 

 Navashin, Heitz, and others on various plants, besides establishing 

 morphological and developmental relationships, indicates possible 

 lines of attack on problems of nucleolar function. 



We may proceed now to a discussion of some features of chromo- 

 some structure for which submicroscopic interpretations are as yet 

 either very limited in scope or else highly speculative. Very diverse 

 opinions have been expressed by different workers on the function 

 or even the existence of the matrix of the chromosomes. Darlington 

 (1935a) dismisses it entirely but concedes that "the chromosome 

 thread probably has some sort of pelUcle." Heitz and Kuwada, respec- 

 tively, introduced the terms kalymma and hyalonema. These and 

 others are, as Nebel (1939) says, "synonymous terms for a morpho- 

 logical entity, at present insufficiently defined in terms of chemical 

 constitution." Some authors, including myself, consider that there 

 is evidence for both matrix and a sheath or pellicle. Nebel considers 

 each chromonema to have its own matrix and all the chromonemata 

 of a chromosome to have, in addition, a common matrix. However 

 this may be, it may be taken as established that the chromosomes 

 in mitosis and meiosis consist of chromonemata surrounded by a 

 substance for which we may as well use the now noncommittal term 

 matrix until more is known of its nature, structure, and function. 



From this point we may logically consider next the problem of 

 the number of chromonemata and their arrangement within the 

 chromosomes during division. As Nebel (1939) has so recently 

 presented an extensive review of chromosome structure, only a brief 

 outline, particularly of those problems still at issue, will be given 

 here. The detailed nature of the coiling of the chromonemata within 

 mitotic or meiotic chromosomes and the mechanisms concerned in 

 coiling have engaged much attention since about 1925 when Kauf- 

 mann demonstrated it clearly in pollen mother-cell meiosis of plants 

 with large chromosomes. More recently spiral structure has been 

 established definitely in somatic chromosomes of plants, in grass- 

 hoppers (White, 1940) , sawflies (Smith, 1941) , a mammal (Roller, 

 1938) , and protozoa (Cleveland, 1938) . Though it has not yet been 

 clearly demonstrated in many animals nor in plants with very small 

 chromosomes, it seems a fairly safe assumption that the condensed 

 chromosomes characteristic of mitosis and meiosis in most organisms 

 all have a coiled structure and that spiralization is the chief mechan- 



