EXPERIMENTAL MODIFICATIONS 77 



also produce a very appreciable decrease (30-40%) in the oxygen 

 consumption (Brachet, 1949a). 



Once again, it is clear that new tools such as electron microscopy 

 and autoradiography should be used in the case of heated amphib- 

 ian gastrulae. Electron microscopy could give very valuable in- 

 formation about the alterations which probably occur in the ultra- 

 fine structure of the mitochondria and the basophilic cytoplasmic 

 constituents. Autoradiography, on the other hand, might throw 

 useful light on the more dynamic aspects of nucleic acid and protein 

 synthesis in heated gastrulae. But, whatever the results given by 

 these new methods, the present conclusion will certainly remain 

 valid. Changes in morphogenesis and in RNA distribution and syn- 

 thesis are always parallel in heated embryos. 



It is a well-estabHshed fact that sublethal cytolysis produced by 

 shifts in the pH or removal of the calcium ions of the medium can 

 provoke the spontaneous neuralization of ectoblastic explants 

 (Holtfreter, 1947). The treatments used by Holtfreter (1947) pro- 

 duce effects which resemble superficially the abnormalities pro- 

 duced by heat shocks, since they are also of short duration and 

 applied at the early gastrula stage. However, heat shocks differ 

 from the acid and alkaline shocks used by Holtfreter (1947) in that 

 they never produce spontaneous neuralization (Mookerjee, 1953). 

 Thus heat shocks decrease the morphogenetic potential of ex- 

 planted ectoblastic fragments, while acid and alkaline shocks often 

 increase it. 



Too little is known as yet about the chemical and ultra-structural 

 changes induced by acid and alkaline shocks for any definite con- 

 clusions to be drawn. All that can be said is that they modify the 

 structure of the cells in much the same way as does the centrifuga- 

 tion of blastulae. As shown in Fig. 28 (p. 74), the RNA-rich cyto- 

 plasm, in the cells which have been exposed to an acid or alkaline 

 medium, accumulates in the form of a basophilic crescent (Brachet, 

 1946). As we have already seen, the local concentration of RNA at 

 one pole of the cells is a characteristic feature of both normal in- 

 duction and formation of additional embryonic axes in centrifuged 

 blastulae. It can thus be supposed that the crescent-shaped accu- 



Referencesp. 90/93 



