292 NUCLEIC ACIDS AND GROWTH 3 



results have been so disappointing that they have not yet been pubhshed. 

 Ribonuclease has no visible effect on gastrulae or explanted dorsal lips in 

 many species; apparently the enzyme does not get into the cells at that stage. 

 With some Urodeles, ribonuclease dissolves the surface coat (Holtfreter, 1943) 

 which unites the cells : the ectoderm is progressively pealed off and induction is 

 of course deficient or absent in these ectodermless embryos. But the failure of 

 induction is due to the destruction of the reacting ectoderm rather than to a direct 

 action on the inductor, since chorda and somites can differentiate normally in the 

 treated embryos. An interesting point, however, is that the disintegrating action 

 of ribonuclease on the surface coat is highest in the regions which are richest in 

 RNA: a clear differential susceptibility is evident in this case. 



(c) The effects of an abnormal nuclear composition on nucleic acids and development 



Abnormalities of nuclear composition (hybridization, haploidy, polyploidy, 

 etc.) lead to interesting abnormalities of development. Since the whole subject 

 has recently been reviewed in detail by Moore (1955), we shall deal only with the 

 very few cases where nucleic-acid metabolism has been studied. 



Of greatest interest are the diploid lethal hybrids in Amphibians : in the lethal 

 combinations Rana esculenta 9 X Rana fiisca -j (Brachet, 1944b) and Rana pipiens ^ 

 X Rana sylvatica cf (Moore, 1946, 1947, 1948), the introduction of the foreign sperm 

 nucleus is followed by normal cleavage; but development stops entirely at the 

 onset of gastrulation and the blocked embryos cytolyze a few days later. 



Just as in the heat-treated eggs, the dorsal lips of the lethal hybrid amphibian 

 gastrulae resume further development and show inducing activity if they are graft- 

 ed into normal embryos (even from a remote species, such as a Urodele). The 

 inducing ability of the lethal hybrid's organizer seems, however, to be somewhat 

 reduced (Brachet, 1944a; Moore 1946, 1947, 1948). It looks as if the presence of 

 a foreign nucleus made it impossible for the hybrid to produce substances neces- 

 sary for further development; these substances easily diffuse from the host to the 

 graft which is "revitalized". 



Cytochemical observations (Brachet, 1944a) have shown that RNA synthesis 

 does not occur in the arrested lethal hybrids, although abnormal amounts of 

 RNA may accumulate in their nuclei (Brachet, 1954b). When a piece of the block- 

 ed gastrula is grafted in a normal host, the basophilia of the nucleoli and the 

 cytoplasm markedly and steadily increases when morphogenesis is resumed: the 

 close link between RNA synthesis and development is especially obvious in this 

 case. These cytochemical observations have been entirely confirmed, in the lethal 

 combination R. esculenta 9 X R. fuse a ^ by the quantitative estimations of Steinert 

 (1951): there is almost no RNA synthesis in the blocked embryo, while the con- 

 trols steadily increase their RNA content. 



Comparable results, in the Urodele combination Triton palmatus 9 X Salamandra 

 atra rf , have been reported by Shen (1954), who even found a drop in the RNA 

 content of the lethal embryos at the time lethality sets in. This drop might be due, 

 as shown by cytochemical observations of Baltzer and Schonmann (1951) and 

 Baltzer (1952), to the fact that many cells die at this stage in the Triton X Sala- 

 mandra hybrids; this is not the case in the Rana hybrids we studied. 



