78 GROWTH AND MORPHOGENESIS 



mulation of RNA-rich cytoplasm in cells which have been sub- 

 mitted to acid or alkaline shocks has something to do with spon- 

 taneous neuralization. 



A good correlation between morphogenesis and RNA synthesis 

 is also found when an egg is fertilized by a spermatozoon belonging 

 to another species with the production of a lethal hybrid, or by 

 more than one spermatozoon (polyspermy). RNA synthesis stops 

 when development of the lethal hybrid is blocked, usually as a 

 gastrula. There is a resumption of this synthesis when a fragment 

 of the lethal hybrid becomes "revitalized" after it has been trans- 

 planted into a normal host. 



Finally, dispermic eggs are often formed of a diploid and a 

 haploid half. If the two halves are equally well developed, their 

 RNA content is the same. But when the haploid half is under- 

 developed, its RNA content is lower than that of the diploid half. 

 In the case of dispermic eggs, it is clear that RNA synthesis is not 

 linked to the diploid or haploid condition per se, but to the degree 

 of morphogenesis which has been attained by diploid and haploid 

 organs. 



In summary^ all the evidence that we have concerning RNA dis- 

 tribution and synthesis in normal and experimental embryos (action 

 of numerous chemical substances, centrifugation, heating, treat- 

 ment with acid or alkali, production of an abnormal nuclear con- 

 dition, etc.) shows that these phenomena are always closely linked 

 to morphogenesis. One should not forget, however, that RNA is 

 only one of the many constituents of ribonucleoprotein particles. 

 There is no proof, for the time being, that RNA is in itself more 

 important for morphogenesis than the proteins and lipids with 

 which it is associated in ergastoplasmic structures. The advantages 

 of RNA over the other unknown constituents of basophilic struc- 

 tures are the relative ease of its cytochemical detection and its im- 

 portant role in protein synthesis. In any event, experiments designed 

 to demonstrate in an unambiguous way the role of RNA itself in 

 morphogenetic processes are highly desirable. 



Brachet and Ledoux (1955) and Brachet (1959) have attempted 

 to attack directly this important problem by treating living am- 



