RNA AND GROWTH 



PROTEIN SYNTHESIS 



283 



Fig. 9. Block of mitotic activity in ribo- 

 nuclease-treated morula of Pleurodeles 

 (note marked swelling of the arrested 

 interphase nuclei). 



Fig. 10. Partial blastula after 4 h. ribo- 

 nuclease treatment and culture in normal 

 medium during 64 h. 



Fig. II. Partial neurula after treatment 

 with a ribonuclease-RNA mixture (4 h.) 

 and culture in normal medium during 

 64 h. 



stages, probably because the enzyme 

 can no longer penetrate into the eggs 

 at later stages. As shown in Fig. 9, 

 mitoses are blocked in interphase, 

 probably because ribonuclease inter- 

 feres with centrosomes or spindle 

 development. Penetration of ribo- 

 nuclease into morulae is slow; if the 

 eggs, after a few hours treatment, 

 are returned to a normal medium, 

 mitosis is resumed in the internal 

 cells which surround the blastocele. 

 These cells may, after a few days, 

 migrate through the irreversibly 

 blocked external blastomeres and 

 form an atypical ectoderm (Fig. 10). 

 If the ribonuclease treatment is made 

 less effective by the addition of RNA, 

 one can observe the formation of a 

 neural tube, laying on large, un- 

 cleaved and undifferentiated blasto- 

 meres (Fig. 11). In amphibian eggs, 

 as in other cells, ribonuclease pro- 

 duces a strong (85%) inhibition of the 

 incorporation of labelled precursors 

 into the proteins: it is worth men- 

 tioning that, according to the auto- 

 radiography observations, protein 

 synthesis is chiefly limited to the 

 nuclei during early cleavage. This 

 synthesis of nuclear proteins is there- 

 fore very sensitive to ribonuclease. 

 Results, which entirely confirm 

 our observations on amoebae, have 



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