Ill PFS OF DIFFERENTIATED AND EMBRYONIC CELLS 517 



scattering between eggs from one clutch reaches the magnitude of scattering 

 observed between different ckitches. The mean RNA value rises during gastrula- 

 tion, accumulation coming to a standstill during neurulation. In the merogonic 

 hybrids [palmatus sperm x cristatus egg cytoplasm) the initial RNA content is 

 lower than in the controls, but in the course of development the rate of accumu- 

 lation in the merogonic hybrids reaches the magnitude of that observed in the 

 controls. Also, the accumulation of RNA in the hybrids continues past the stage 

 of development during which RNA accumulation in the controls has come to 

 a standstill. Therefore, the hybrids acquire, with time, a higher RNA content than 

 the controls. Throughout the observed course of development the scattering of 

 the RNA values in the hybrids was found to be restricted to a narrower range than 

 in the controls. As an interesting interpretation of these results, Zeller suggests that 

 the RNA, present in the egg before fertilization, is degraded in the hybrid more 

 rapidly, giving rise to the initially lower RNA values and also to a lower scattering. 

 The newly synthesized RNA, produced under the control of the cristatus nucleus 

 cannot be degraded in the pabnatus cytoplasm and therefore the accumulation 

 continues longer than in the controls. It is concluded that hybridization, in this 

 case, interferes mainly with the breakdown of the newly synthesized RNA. Further 

 investigation seems warranted to examine the possibility that the final lethal 

 effect of this hybridization is due to the absence of RNA turnover and a subsequent 

 inhibition of new protein formation which depends upon a constant replenishment 

 of RNA as in the case of induced enzymes in bacteria. 



In considering nucleocytoplasmic relations, experiments should be mentioned 

 which show that that caryoclastic agents like nitrogen mustard exert two different 

 effects; the one, consisting in a direct interference with mitosis; the other becoming 

 manifest in cells at an early state of differentiation such as neural tissue shortly 

 after induction (Nieuwkoop and Lehmann, 1952). The two types of action have 

 been sharply differentiated by Bodenstein and Kondritzer (1948) in experiments 

 with Drosophila. In the former case one finds inhibition of mitosis with continuation 

 of cytoplasmic synthesis, continued RNA synthesis, and formation of giant cells 

 (Bodenstein and Kondritzer, 1948). In the other case interference with the nuclear 

 activity leads to a complete degeneration of the cell. Similarly, Gelfant et al. (1955) 

 could show that under conditions of mitotic arrest by aminopterin and nitrogen 

 mustard the response of the uterine epithelium (increase in cell size) to hormonal 

 stimulation is not inhibited. In contrast, the growth of the muscular layer after 

 hormonal stimulation seems Lo depend on nuclear proliferation and is inhibited by 

 the antimitotic agents investigated. Increased incorporation of methionine--^^S 

 (Sirlin, 1955) and glycine- i-^'^C (Sirlin and Waddington, 1954) into nuclei of the 

 dorso-axial structures and of the induced neural plate was observed radioauto- 

 graphically and has been interpreted as an indication of a higher nuclear metabolism 

 which is possibly related to the nucleo-cytoplasmic interactions discussed above. 



Although in cytological and chemical work more attention is given, at the 

 present time, to the effect of nuclear factors upon cytoplasmic activities, the reverse 

 relationship must also be considered in view of the experimental evidence cited 

 above. Chemical evidence for cytoplasmic effects on the nucleus during embryonic 

 development seem to be lacking and for this reason a recent suggestion made by 



Literature p. sjg 



