THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 



255 



of blastulae. As shown in Figure 11, the RNA-rich cytoplasm, in cells 

 that have been exposed to an acid or alkaline medium, accumulates in 

 the form of a basophilic crescent ( Brachet, 1946 ) . As we have already 

 seen, the local concentration of RNA at one pole of the cells is a char- 

 acteristic feature of both normal induction and formation of additional 

 embryonic axes in centrifuged blastulae; it can therefore be supposed 

 that the crescent-shaped accumulation of RNA-rich cytoplasm in cells 

 that have been submitted to acid or alkaline shocks has something to 

 do with spontaneous neuralization. 



A good correlation between morphogenesis and RNA synthesis is 

 also found when an egg is fertilized with a spermatozoon belonging to 

 another species and produces a lethal hybrid, or is fertilized with more 

 than one spermatozoon (polyspermy). RNA synthesis stops when de- 

 velopment 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 transplanted into a normal host 

 (Brachet, 1944, 1957). 



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

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

 content is the same, but when the haploid half is underdeveloped, its 

 RNA content is lower than that of the diploid half. In the case of dis- 

 permic eggs, it is clear that RNA synthesis is linked not to the diploid 



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Figure 11. RNA-rich, basophilic crescents forming in ectoderm cells which 

 have been exposed to an alkaline shock (Brachet, 1957) . 



