EXPERIMENTAL MODIFICATIONS 79 



phibian eggs with ribonuclease. It was hoped that the enzyme might 

 penetrate into the Hving cells, inactivate or break down the RNA 

 which they contained and exert important morphogenetic effects. 

 As we shall see, these hopes have not been entirely fulfilled because 

 of the poor penetration of ribonuclease into amphibian eggs once 

 cleavage is over. 



We have already seen that ribonuclease quickly inhibits cleavage 

 in amphibian eggs and that the nuclei are usually blocked in inter- 

 phase. The penetration of the enzyme is, however, slow and in- 

 complete. Therefore, only the blastomeres which form the outer 

 layers of the morula are irreversibly blocked in their development. 

 If the treated morulae are returned to the normal medium, after a 

 few hours of treatment with ribonuclease, the innermost blastome- 

 res which surround the blastocele resume cleavage. They finally 

 migrate through the dying or dead outer blastomeres and form an 

 atypical undifferentiated ectoderm (Fig. 29, p. 75). If the eggs are 

 treated with a mixture of ribonuclease and RNA, or if they are 

 placed in a RNA-containing medium after the ribonuclease treat- 

 ment, one can occasionally obtain the formation of a nervous 

 system (Fig. 30, p. 75). It lies on a bed of large, blocked cells. The 

 experiments are, unfortunately, not reproducible enough to allow 

 definite and general conclusions. However, the fact that we have 

 never obtained a nervous system after treatment with ribonuclease 

 alone, but obtained several ''neurulae" after a ribonuclease-RNA 

 treatment, provides a definite indication of a role for RNA in normal 

 induction. 



Penetration of ribonuclease at later stages of development is 

 usually very poor and thus little or no effect on morphogenesis is 

 observed. One can, however, occasionally find ribonuclease prep- 

 arations which are more active than others. Since their effects can 

 be duplicated by adding small amounts of versene (EDTA) to 

 otherwise inactive preparations of ribonuclease, it is probable that 

 the active preparations contain some chelating agent as a contami- 

 nant. 



When blastulae or gastrulae are treated with ribonuclease rein- 

 forced by the addition of versene (at low concentrations at which 



References p. 90/93 



