70 GROWTH AND MORPHOGENESIS 



Biochemical and cytochemical work by Lallier (1954) and by 

 Thomason (1957) has clearly shown that lithium interferes with 

 RNA distribution and synthesis in amphibian eggs. According to 

 Lallier (1954), lithium decreases the RNA gradients, whereas in 

 Thomason's (1957) work, this ion was found to inhibit markedly 

 the incorporation of ^^P into the nucleoprotein fraction. 



It would, however, be an over-simplification to believe that 

 lithium acts in a specific way on nucleic acid distribution and syn- 

 thesis. For instance, Lallier (1954) has observed that lithium inhibits 

 the dehydrogenases of the tricarboxylic acid cycle, while Ranzi 

 (1957a, b) has produced a considerable amount of evidence for the 

 view that lithium radically modifies the physical properties of the 

 fibrous proteins present in the egg. 



We have seen, in the foregoing, that RNA synthesis and distri- 

 bution on the one hand and morphogenesis on the other appear 

 to be very closely linked processes in developing amphibian and 

 chick eggs treated with a great variety of chemical inhibitors. We 

 shall now consider the effects of physical treatments, such as cen- 

 trifugation, heating or changing the pH of the surrounding me- 

 dium, on the RNA gradients. 



Centrifuging developing eggs is an easy way to modify both the 

 gradient distribution of substances or cell organelles, and morphogen- 

 esis. The most important experiments in this field are those of 

 Pasteels (1940, 1953), who worked with amphibian eggs. He found 

 that centrifugation of freshly fertilized eggs leads to the formation 

 of "hypomorph" embryos. They show deficiencies in the nervous 

 system which may go from complete absence to strong micro- 

 cephaly. When gastrulation of these centrifuged embryos is normal, 

 the result is the production of embryos which have an almost 

 normal tail, but practically no head. On the other hand, centrifu- 

 gation of blastulae leads to the formation of double or even triple 

 embryos (Pasteels, 1953). 



Unpublished studies of Pasteels and Brachet have shown that the 

 centrifugation of both freshly fertilized eggs and blastulae produces 

 profound changes in the distribution of RNA. As shown in Fig. 24 

 (p. 64), ribonucleoproteins accumulate at the animal pole when 



