CELL NUCLEUS AND MORPHOGENESIS 87 



which occur in a normal embryo. It is important to remember, in 

 this respect, that Holtfreter (1955) found that killed tissues liberate 

 substances which can diffuse through a cellophane membrane and 

 induce the neuralization of ectoblast fragments in contact with the 

 membrane. The killed tissues of Holtfreter (1955) obviously behave 

 quite differently from the living organizer, whose inductive power 

 is stopped by the insertion of a cellophane membrane (Brachet, 

 1950). 



6. THE ROLE OF THE CELL NUCLEUS IN MORPHOGENESIS 



There is no doubt that in all vertebrates a very important factor in 

 m.orphogenesis is the existence of a precise gradient pattern, which 

 involves both RNA (ergastoplasm) and mitochondria. Since RNA 

 and the ATP produced by the mitochondria are two major factors 

 in protein synthesis, it is clear that the above-mentioned animal- 

 vegetal and dorso-ventral gradients must also be gradients of pro- 

 tein synthesis. Autoradiography, especially in the work of Tencer 

 (1958), confirms this expectation. However, dynamic studies on the 

 incorporation of various precursors into nucleic acids and proteins 

 do not give exactly the same picture as the more static methods 

 used for the cytochemical detection of RNA. As already mentioned, 

 they show that incorporation of these precursors is very much 

 greater in the nuclei than in the cytoplasm during cleavage. It 

 appears as if the main job for the egg is to undergo mitoses as fast 

 as possible during this initial phase of development. The result is 

 that synthetic processes are almost reduced to the reduplication of 

 the constituents of chromatin itself. Later on, at gastrulation, syn- 

 thesis of RNA and proteins becomes evident in the cytoplasm as 

 well as in the nuclei. This synthesis follows the very distinct dorso- 

 ventral and antero-posterior RNA gradients found in the gastrula 

 and the neurula (Tencer, 1958). 



It was suggested by the author, as early as 1949, that the changes 

 in metabolism which occur at gastrulation are linked to a new type 

 of interaction between the nucleus and the cytoplasm. During 

 cleavage, biochemical interactions between nuclei and cytoplasm 



References p. 90/93 



