482 J. BRACKET 



are no less impressive in showing the importance of PN A in morphogenesis : 

 if frog gastrulse are heated at 36.6° for 1 hr., their development is entirely 

 blocked, although they do not cytolyze until 2 to 3 days later (Brachet^"'^^). 

 At a slightly lower temperature (36.2°), a similar heat shock produces only 

 a temporary, reversible block of development. In the first case, cytochemi- 

 cal methods for the detection of PNA indicate that its synthesis is com- 

 pletely arrested, a fact which has been confirmed by the chemical investiga- 

 tions carried out by Steinert.^^ In the case of a reversible shock, PNA 

 synthesis is only temporarily stopped and is resumed as soon as develop- 

 ment starts again. It has been further shown (Brachet^^'^^) that, if a frag- 

 ment of a heated gastrula (irreversibly blocked) is grafted into a normal 

 embryo, it differentiates normally after an initial lag period. As soon as 

 the "revitalization" process sets in, the basophilia of the heated cells begins 

 to increase. 



One of the main actions of heat shocks is to affect the microsomes; these 

 undergo a partial denaturation and lose most of their PNA (Brachet^^). 

 In the heat-treated eggs, a larger proportion of the PNA is no longer bound 

 to the microsomes but is to be found in the supernatant fraction after 

 ultracentrifugation. However, it should be pointed out that the heat shock 

 does not affect the microsomes in a specific manner: the oxygen consump- 

 tion of the heated embryos is reduced by 30 to 40 % (Brachet^*) and their 

 DNA has already undergone changes in its structure (Thomas^^), as indi- 

 cated by an increased ultraviolet absorption. 



d. Abnormal Nuclear Corn-position 



A situation essentially similar to that just described prevails in am- 

 phibian eggs which have been fertilized by abnormal sperm: hybridization 

 with sperm of a foreign species or fertilization with sperm treated with 

 nitrogen mustard both lead to a highly lethal condition: cleavage of the 

 eggs is perfectly normal, but development ceases entirely at the onset of 

 gastrulation and the blocked lethal hybrids disintegrate a few days later. 



We shall deal briefly here with only one of the many lethal hybrids which 

 have been studied, the lethal combination Rana esculenta 9 x Rana fusca 

 d^, because it is the only one in which the behavior of the PNA is fairly 

 well known (Brachet^^*"). 



Just as in the heat-treated eggs, parts of the blocked lethal embryos can 

 be "revitalized" and will develop further if they are grafted in normal 



36 J. Brachet, Experientia 4, 353 (1948). 



" J. Brachet, Pubbl. staz. zool. Napoli 21, 77 (1949). 



38 J. Brachet, Bull. soc. chim. biol. 31, 724 (1949). 



39 R. Thomas, Experientia 7, 261 (1951). 



'"' J. Brachet, Ann. soc. roy. zool. belg. 75, 49 (1944). 



I 



