Ill PFS OF DIFFERENTIATED AND EMBRYONIC CELLS 515 



been cogently pointed out in a recent review by Schultz (1953). Investigations of 

 nucleo-cytoplasmic relations of particular pertinence from an embryological 

 point of view have been summarized by Fankhauser (1952 and 1955) and by 

 Briggs and King (1955). Beginning with the work of Roux, Weisman, Boveri and 

 continuing with the investigations of Baltzer and Hadorn and leading up to the 

 present cytochemical studies of Caspersson and others, the effects of cytoplasm on 

 the nucleus have received alternatingly greater and lesser emphasis. The role of 

 the nucleus in embryonic development is borne out by the interesting experiments 

 carried out by King and Briggs ( 1 955) . Nuclei were isolated from cells of amphibians 

 {Rana pipiens) at early and late gastrula stages and implanted into enucleated eggs. 

 While early gastrula nuclei maintain development through larval stages, implan- 

 tations of late gastrula nuclei lead to a developmental arrest at the blastula, 

 gastrula, or neurula stage. The nuclear differentiation accompanying spermato- 

 genesis has been explored recently with elegant histochemical methods by 

 Alfert (1956). 



The bulk of chemical evidence for a nucleo-cytoplasmic interaction has centered 

 upon the demonstration of a transfer of RNA from the nucleus to the cytoplasm. 

 The speculation seemed attractive that RNA might act as a mold in transferring 

 structural properties of genetic determinations. 



More recently direct evidence for the transfer of nuclear RNA to the cytoplasm 

 has been established (Goldstein and Plant, 1955). Cell constituents of amoebae 

 are labelled with ^^p by feeding them Tetrahymena containing this tracer. Under 

 the conditions of the experiment (arrested proliferation) the nucleus of the amoe- 

 ba incorporates the label only into its RNA molecules. A labelled nucleus was 

 removed and transferred to another amoeba and the distribution of the label from 

 the nucleus of the donor into the cytoplasm of the host was followed radioauto- 

 graphically. For five hours no label left the nucleus but after 1 2 hours a consider- 

 able portion of the labelled material had penetrated into the cytoplasm of the host 

 amoeba and was still present in the form of RNA (RNase digestible) . Since an 

 unlabelled nucleus placed into the cytoplasm together with the labelled nucleus 

 did not show any uptake of the label, the transport of the RNA seems to be possible 

 only from the nucleus into the cytoplasm and not in the opposite direction. 

 Transfer of nuclear material has been observed in the electron microscope in the 

 form of characteristic nuclear blebs which are released into the cytoplasm coincident 

 with secretory activity in salivary gland cells oi Drosophila (Gay, 1955 and 1956). 



To what extent protein formation in the cytoplasm of different cell types depends 

 on the supply of nuclear RNA is still undecided. The early decrease in incorpora- 

 tion into cytoplasmic proteins observed by Mazia and Prescott (1955) immediately 

 after enucleation of amoebae would seem to suggest strongly that the nucleus does 

 participate in the maintenance of the protein forming capacity of the cytoplasm. 

 Similar experiments in Acetabularia have led to a decay of cytoplasmic protein 

 formation only after a longer period of enucleation (Brachet, Chantrenne and 

 Vanderhaeghe, 1955). 



Mazia and Prescott also suggest the possibility that protein formed in the nucleus 

 may become associated with nuclear RNA and be transferred to the cytoplasm 

 when RNA is released from the nucleus. Some protein forming capacity of the 



Literature p. 5jp 



