4IO 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



RIGHT 

 SIDE 



^ _a (Node level) 



□ nii 



Fig. 71. A map of induction frequency of the perinodal area at the primitive streak 



stage of the chick. The various pieces have been isolated and inserted between two layers 



of a host blastoderm cultivated in vitro. From Mulherkar, 1958. 



At the end of this rather extensive, but not exhaustive survey of comparable 

 experiments, a few points may be stressed. Before gastrulation, the most active 

 region of the Urodele egg is only able, as such, to notogenesis. The Anuran egg 

 has probably passed through a similar stage but has already developed a pre- 

 chordal anlage with specific potency. This step is made by the Urodele egg while 

 it begins to gastrulate, and could well be correlated with the kinematics of 

 invagination. Gradually, the anterior part of the archenteric roof acquires the 

 morphological and dynamic features of the prechordal plate. At this stage the 

 main system of inductive processes may be considered double: headward, the 

 prechordal endoblast and mesoblast; behind, the chordomesoblast. Several 

 results show the formation of acrencephalic organs in conditions which suggest 

 the neoformation of prechordal material from adjacent chordomesoblast. Other 

 experiments cause the same brain parts and sense organs to appear by an ap- 

 parently diflferent mechanism, called "activation". 



During the final steps of morphochoresis, when mesoblastic mesenchyme is 

 produced, its cells acquire a new type of influence on the spinal cord and probably 

 on the whole neural axis, i.e., their presence limits the proliferation of the neuro- 

 blasts located in the germinative layer and controls, perhaps indirectly, the 

 expansion of the ependymal lumen. For those who know the advances which 

 have been made in the study of induction, the striking innovation is the idea that 

 the inductive system is not a preformed complex capable of simply unfolding 

 itself but a device which becomes more complicated along with the course of 

 morphochoresis. This chronological transformation is the discovery of Y. K. 

 Okada and his students. 



