118 The Structure of Protoplasyn 



ploid as well as diploid cells of Drosophila, the homologous chromo- 

 somes all tend to lie together. 



The fact that endomitosis provides a way in which the nucleo- 

 protein content of a nucleus may be increased, and the apparent 

 association of endomitotic chromosome or chromonema multiplication 

 with secretory or nutritional function have naturally led to a good 

 deal of speculation on the mechanism of the exchange which must 

 take place between the nucleus and the cytoplasm. Without going 

 seriously into this problem an indication of the present state of opin- 

 ion may be given by citing three papers which appeared in close 

 succession in the same Journal last year. Painter (1940) suggests 

 that rapidly segmenting eggs can draw the material for the synthesis 

 of their chromosomes from the chromatin or its derivatives that have 

 been produced by endomitosis and poured out into the cytoplasm by 

 the breakdown of the germinal vesicle, or by indirect absorption from 

 nurse cells. Calvm, Kodani, and Goldschmidt (1940) consider that 

 salivary gland chromosomes are essentially of the same structure 

 as mammalian egg "lampbrush" chromosomes from which they con- 

 sider chromatin is being sloughed off. Caspersson and Schultz 

 (1940) , however, point out that the high concentration of ribonucleic 

 acid in the cytoplasm occurs before the nuclear membrane breaks 

 down. They observe that the cytoplasm or nucleolus which does not 

 ordinarily stain with Feulgen does contain nucleic acids which must 

 be assumed to be of the ribose type. There is a high concentration of 

 these around the nuclear membrane in several rapidly growing 

 tissues and a gradient from it to the outside of the cell. The amount 

 of ribonucleic acid in the cytoplasm can be increased by adding 

 heterochromatin (in particular by adding extra Y chromosomes in 

 Drosophila) . It seems that more must be learned of the relationships 

 between different nucleic acids before we can proceed much further 

 with this problem. The same applies to the relationship between the 

 chromosomes and the nucleolus. Of the latter, Nebel (1939) says, 

 "The understanding of this problem has not progressed far during 

 the last fifty years" and "recent work has further emphasized the 

 complexity of the problem in that it is necessary to avoid all general- 

 izations." Caspersson and Schultz commit themselves to little in 

 this connection except to state that nucleoli are produced in hetero- 

 chromatic regions, that these regions "are especially concerned with 

 the ribonucleic acid metabolism of the cell," and that "activity of 

 nucleoU is closely associated with an intense synthesis of the cyto- 

 plasmic ribonucleic acids." Since the heterochromatic regions stain 



