Structural Differentiation of the Nucleus 115 



capacity " (1) to form large amounts of thymonucleic acid (or. better 

 perhaps, thymonucleoprotein) in the chromosomes themselves; (2) 

 to form or affect the composition of the nucleoli; (3) to affect the 

 characteristics of neighboring regions translocated to them in such 

 a way as to change the developmental effects of these regions in 

 somatic cells; (4) to affect the content of the ribonucleic acids in the 

 egg cytoplasm of Drosophila." Conclusions (1), (2), and (4) are 

 based largely on ultraviolet data. Number (3) is a conclusion 

 drawn jointly from submicroscopic and genetic data; the problem 

 of the mechanism of the developmental effect, viz., variegation, 

 remains open and need not be entered upon here. Conclusion (4) 

 leads us to the problem of the transfer of materials between the 

 nucleus, nucleolus, and cytoplasm, which will be discussed after we 

 have considered ways of increasing the nucleic acid content of a 

 given nucleus by changing its chromosome constitution. 



The old problems of the nucleo-plasma ratio and of changes in 

 nuclear volume began to receive renewed attention following the 

 observations of Jacobj (1935) on differences in nuclear size within 

 a tissue. He found that there were regular differences in size that 

 did not follow the probability curve but one with several maxima. 

 One maximum tended to be about double that of the preceding one, 

 i. e., nuclear volumes of 1:2:4:8 were more frequent than inter- 

 mediate values. In man almost all tissues were foujid to have nuclei 

 in nine classes, with volumes ranging from 1-256, which would 

 involve eight doublings. To account for this, Jacobj suggested 

 rhythmic growth, an alternation of longer rest periods and shorter 

 growth periods. Wermel also found a rhythm of nuclear growth in 

 tissue cultures, but showed that the volume doubling was not 

 reached in a single period of growth; the increase was first to one 

 and one-half times. Similar results were obtained in observations 

 on nuclear growth in the intestine of Anopheles and in the fat bodies 

 of silkworms (see Wermel and Portugalow, 1935). Fischer (1935) 

 found that growth of the follicular epithelium of some Orthoptera 

 occurs in two ways: (1) by mitotic increase in cell number, during 

 which the nuclei remain in the smallest size class, (2) by increase 

 in cell volume, with constant cell number, which occurs when the 

 secretory function of the cell starts. He states that the secretory 

 cells of the follicle do not divide mitotically at this time but that 

 after the last mitosis there is a doubling of nuclear volume and then 

 what is apparently an amitotic division. After this there is rhythmic 



