246 CELLS, TISSUES, AND ORGANISMS 



Figure 4. Distribution of RNA in a late amphibian gastrula. Note the 

 stronger basophilia in the dorsal (right) than in the ventral (left) half 

 (Brachet, 1957). 



act with each other. The outcome is the appearance, in the late gastrula 

 and the early neurula ( Figure 5 ) , of very well-defined antero-posterior 

 ( cephalo-caudal ) and dorso-ventral gradients; the latter is especially 

 apparent in the chordomesoblast. 



When sections of late gastrulae or early neurulae are examined 

 under high power, a high RNA content is found at the boundary sepa- 

 rating the young medullary plate from presumptive chorda. It looks as 

 if, at the very time of induction, RNA accumulates precisely at the 

 points where inductor and reacting system are in close contact. Similar 

 observations have been made more recently in chick embryos (Lava- 

 rack, 1957). 



At later stages of embryonic development, the RNA content of 

 every organ increases just before its dififerentiation begins. DifiFerentia- 

 tion itself ( for instance, vacuolization of the notochord or formation of 

 neurones in the nervous system) often results in a drop in the RNA 

 content of the individual cells, except when the latter belong to an 

 actively-protein-synthesizing organ ( the liver or pancreas, for instance ) . 



Enough experimental work has been done to demonstrate the 

 reality of these gradients (Brachet, 1942; Steinert, 1951; Takata, 1953; 

 Flickinger and Blount, 1957 ) . In particular, the work of Flickinger and 

 Blount, who used tracer methods with radioactive phosphorus 32, leads 



