258 



CELLS, TISSUES, AND ORGANISMS 



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

 latter is inactive by itself), the ectoderm cells dissociate, lose their 

 basophilia, and finally cytolyze (Brachet, 1959). The consequence is 

 the formation of "ectodermless embryos," which have well-differenti- 

 ated chorda and somites but no nervous system or a very reduced one 

 (Figure 14). It was also found that cells of the organizer exhibit a 

 marked differential susceptibility toward the ribonuclease-versene mix- 

 ture: an explanted organizer cytolyzes much more quickly in this 

 medium than explanted ventral mesoblast does. 



The fact that it is possible, after gastrulae have been treated with 

 ribonuclease and versene, to obtain embryos which have no nervous 

 system should not be taken as a proof that RNA is necessary for in- 

 ductive processes in the normal organizer: in the experiments just 

 described, the absence of induction is simply due to the peeling off of 

 the reacting ectoderm cells. 



Figure 14. Ectodermless embryo after treatment of a young gastrula by a 

 mixture of ribonuclease and versene. A chorda has differentiated, but the 

 ectoderm cells are blocked and cytolysing (Brachet, 1959) . 



