ago NUCLEIC acids and growth 3 



(hypomorphic embryos). On the other hand, centrifugation of blastulae leads to 

 the formation of double embryos; again the duplication can be linked to abnormal 

 distribution and synthesis of RNA (Brachet and Pasteels, in Brachet, 1952b). 



The study of effects of heat shocks on morphogenesis and RNA synthesis also 

 indicates a close correlation between the two processes : heating frog gastrulae at 

 36.6° for I h. irreversibly stops development, although the embryos do not cytolyze 

 until 2 or 3 days later; heating at a slightly lower temperature (36.2°) produces 

 only a temporary block of development, followed by the appearance of morphogen- 

 etic abnormalities (Brachet, 1948, 1949a). Cytochemical observations (Brachet, 

 1948) and chemical quantitative estimations (Steinert, 1953) show that RNA 

 synthesis is completely stopped after an irreversible heat shock ; when the effects of 

 heating are reversible, RNA synthesis is resumed as soon as development starts 

 again. Furthermore, if a fragment of an irreversibly blocked gastrula is grafted into 

 a normal embryo, it differentiates again after a lag period; simultaneously the 

 basophilia of the heated cells begins to increase. It seems that heating destroys 

 substances necessary for morphogenesis and that these substances could diffuse 

 from the normal host to the graft. Biochemical observations (Brachet, 1949b) 

 have shown that the RNA-containing microsomes are especially sensitive to 

 heating: in the heated gastrulae, a large proportion of the RNA is no longer 

 bound to the microsomes and is found in the supernatant fraction after ultra- 

 centrifugation. 



These observations clearly indicate that morphogenesis is closely linked to RNA 

 distribution and synthesis in developing embryos. But it would be, as already 

 mentioned, an error to believe that RNA is the only factor involved in the experi- 

 ments which have just been mentioned: it should be emphasized once more that 

 RNA is only a constituent of complex cytoplasmic particles, the microsomes; 

 their main function, as far as we know, is protein synthesis (Brachet and Jeener, 

 1944; Allfrey et al., 1953; PoUister, 1953). RNA alone is unable to synthesize 

 proteins; similarly RNA alone can not induce morphogenesis, i.e. act as an 

 organizer. 



The erroneous view that RNA plays the leading role in the induction of nervous 

 systems by abnormal inductors has been put forward by Brachet (1944a, 1945) 

 on the basis of the following experimental findings : ribonucleoproteins, especially 

 after alcohol treatment, are very good inducing agents when they are grafted into 

 amphibian gastrulae; this inducing activity is considerably reduced if they are 

 treated with ribonuclease prior to implantation. Later work by Kuusi (1953), 

 Brachet, Gothic and Kuusi (1952) and, more recently, by Yamada and Takata 

 (1955) has confirmed that ribonucleoproteins (microsomes or tobacco mosaic virus) 

 are excellent inducing agents; but, in these new experiments, it was found im- 

 possible to abolish the inducing activity by ribonuclease treatment (see also Eng- 

 lander et al., 1953). Since the ribonuclease used by Brachet in his early experi- 

 ments (1944a, 1945) was a rather crude preparation and since Toivonen (1949) 

 and Yamada and Takata (1955a) have found that a slight digestion of the pro- 

 tein part of the ribonucleoprotein with trypsin or chymotrypsin is enough to com- 

 pletely inhibit the induction, there is little doubt that the effects observed earlier 

 were due to partial proteolytic degradation of the implants, rather than to removal 



