4i6 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



Fig. 78. Early consequences of centrifugating a Xenopus blastula. The enibryos have been 



fixed in Zenker and stained with toluidine blue. The dark parts are rich in RNA. (a) An 



axial section of the blastula; (b, c, d) successive neurula stages. In each case, the primary 



system with its archenteron is above. See text. From Pasteels, 1953. 



Pasteels has met this objection with several arguments. The localization of secondary 

 formations does not always fit the induction hypothesis; the period of effectiveness is much 

 more limited than the induction processes; the secondary embryos are not of the type 

 obtained by induction, since they regularly lack an acrencephalon, they show irregularities 

 or desequilibria between the components, and they form mixtures of tissues more similar 

 to the results of xeno-inductions. The explanation of Yamada's failure with centrifuging 

 isolated pieces of ectoblast is probably that the tissue is not submitted to the same physical 

 conditions as when it falls onto the resilient floor or the blastocoele. Consequently, it is 

 legitimate to stress that this thorough investigations has provided us with valuable in- 

 formations. On the one hand, it teaches us that the inherent autonomous potency of the 

 ectoblastic territory is to perform notogenesis, but not acrogenesis, which needs some 

 complementary condition (p. 472). On the other hand, the initial steps of the process show 

 that a thick sheet of ectoblast may produce mesoblastic structures by delamination. This 

 is a kind of pseudogastrulation, a possibility which must not be ignored. 



For the problem of induction, as for other biological analyses, abnormal tempera- 

 tures have been used, at extra- and intra-liminal levels. 



Extra-liminal cold or warmth have proved not only to stop life processes, but often 

 also to modify inducing capacity. It was early recognized that killed tissues, no 



