464 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



can be seen that the fiher is locally attacked by penetration of organic material containing 

 mucoproteins (Fig. 96, c) and RNA. Correlatively, trans-inductions are more successful 

 with filters possessing the larger pores. On the other hand, the cytological character of the 

 two inductions, which are sharply localized, cannot be readily explained by a long-range 

 movement of freely dissociated molecules. Moreover, such a diffusion process would not 

 be restricted by barriers of the lowest porosity range used, which give less favorable results. 

 Thus, this hypothesis must be excluded, but an opposition of large areas of the cell surfaces 

 must also be discarded. Therefore "something intermediate between full cellular contact 

 and free diffusion is suggested". In this way, the author is led to propose the intervention 

 of an intercellular matrix (Fig. 96, d), an idea worth consideration from a more general 

 standpoint. Part of this cautious interpretation may be modified today. Indeed, in the 

 case of metanephrogen mouse tissue submitted to spinal chord, with interposition of filters, 

 it was now convincingly shown that contact is not necessary to obtain a clearcut 

 induction. The use of T-V filters of low porosity, 20 [jl thick, authorizes this conclusion 

 (Grobstein, 1957). The range of maximal distance attains 80 \i, and is in good agreement 

 with the measurements made by Cambar in amphibian embryos (p. 352). In a further 





OIRTCr Ct^.LjLiR 



LONG-RANGE INTEHACtON 



ATRIX INTERACTION 





Fig. 96. Role of mesenchyme in the differentiation of epithelium, (a) A section through a 

 culture in which autogenous mesenchyme was placed under a filter (clear space) and the 

 salivary epithelium above, (b) Section stained with toluidine blue through a filter (left) 

 supporting a culture of spinal cord. The stain penetrates through the surface layer of the 

 filter, and ceases abruptly at the limit of the explant (not shown), (c) Surface view of the 

 spot on a filter where cultivation took place. The PAS reaction gives a purplish color, while 

 adheringfragments of tissue are stained deep blue, (d) Schematic representation of assumed 

 or demonstrated mechanisms of tissue interaction and induction in development; A, B, C, 

 three hypothetical cases of matrix interaction. From Grobstein, 1955a. 



