RNA IN NORMAL AMPHIBIAN EGGS 65 



separating the young medullary plate from the presumptive chorda. 

 It looks as if at the exact time of induction, RNA accumulates pre- 

 cisely at the points where the inductor and reacting system are in 

 close contact. Similar observations have also been made more re- 

 cently in chick embryos (Lavarack, 1957). 



At later stages of embryonic development, the RNA content of 

 every organ increases just before its differentiation begins. Differ- 

 entiation 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-synthesizing organ (liver or pancreas, 

 for instance). 



Considerable experimental work would be required in order 

 to demonstrate the reality of these gradients — which superimpose 

 themselves on the morphogenetic gradients of the experimental 

 embryologists — quantitatively by means of biochemical methods. 

 The work which has already been done in that direction is suf- 

 ficient, however, to leave no doubt concerning the existence of 

 the gradients which have been detected by cytochemical methods 

 (Brachet, 1942; Steinert, 1951; Takata, 1953; Flickinger and 

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

 (1957), who worked with tracer (^^p) methods, leads to the important 

 conclusion that new RNA is being synthesized in morphogenetically 

 active regions during differentiation. 



It should be pointed out, however, that gradients similar to those 

 which have just been described for RNA have also been detected 

 and described for other substances. Sulfhydryl groups bound to the 

 proteins (Brachet, 1940) and reducing systems (Piepho, 1938; 

 Fischer and Hartwig, 1936; Child, 1948) in particular follow the 

 now familiar dorso-ventral and antero-posterior gradient pattern 

 in their distribution. The same pattern has also been found for 

 oxygen consumption (Sze, 1953), the incorporation of amino acids 

 into proteins (Eakin et aL, 1951) and the incorporation of ^^COg 

 into nucleic acids and proteins (Flickinger, 1954). Since mitochon- 

 dria play a leading part in all processes linked to energy production, 

 it appears likely that these cell organelles are distributed, together 



References p. 90/93 



