254 CELLS, TISSUES, AND ORGANISMS 







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Figure 10. The organizer of a gastrula blocked by a heat shock has been 

 grafted into a normal host. It has differentiated into chorda and intestinal 

 lumen and has induced somites and neural masses (Brachet, 1957) . 



Quantitative estimations of the RNA content (Steinert, 1951; 

 Hasegawa, 1955) have shown that, as suggested by cytochemical ob- 

 servations, RNA synthesis is completely inhibited in the irreversibly 

 blocked gastrulae; if the heat shock is too severe, cytolysis begins after 

 three days and the RNA content begins to drop. 



However, it is clear that new tools, such as electron microscopy 

 and autoradiography, should be used in the case of heated amphibian 

 gastrulae; electron microscopy could give very valuable information 

 about the alterations that probably occur in the ultrafine structure of 

 the mitochondria and the basophilic cytoplasmic constituents. Auto- 

 radiography, on the other hand, might throw useful light on the more 

 dynamic aspects of the synthesis of nucleic acid and protein in heated 

 gastrulae. But whatever the results given by these new methods, the 

 present conclusion will certainly remain valid: changes in morpho- 

 genesis and in RNA distribution and synthesis always run parallel in 

 heated embryos. 



It is a well-established 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 (Holt- 

 f refer, 1947). Too little is known as yet about the chemical and ultra- 

 structural changes induced by those acid and alkaline shocks to draw 

 any definite conclusions. All that can be said is that they modify the 

 structure of the cells in much the same way as does the centrifugation 



