320 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4. 



this ability to adapt efficiently comes to an end just when the active period of 

 kinenaatics and layer formation is over and cytodifferentiation is about to take 

 place. This change is currently referred to as determination, a concept which 

 needs some immediate comment. Determination, sensu stricto, means the impos- 

 sibility of changing the fate of cells which have developed beyond a certain 

 critical stage. It has been recognised that, although '"determined"" cells breed true, 

 they still possess a rather limited potential repertoire of modulations (Weiss, 1953) 

 when exposed to different conditions. It is also well known that in an embryo the 

 decisive stage is not attained simultaneously throughout the whole system. Instead, 

 there arise differences, so to speak, in physiological age. Secondary fields of morpho- 

 choresis, like limb buds, tail buds, imaginal discs, echinoid rudiments and similar 

 systems, which are formed later in development, are still capable of regulation 

 or reorganization when other parts of the embryo appear to be fully determined. 



Now, there is a theoretical possibility (Harrison, 1933) that an adequate 

 medium could change the characteristics of cells beyond the recognized limits 

 of modulation. There is even one group of recent experiments which apparently 

 show that such may be the case if an amphibian neurula with closing neural folds 

 is submitted to appropriate antisera (Inoue^ and Ishikawa, 1956). Under this 

 influence, presumptive neural, chordal, somitic, and pronephritic cells regress to 

 a "lower" level of differentiation. This means that in the case of the amphibian 

 neurula, the time of "determination" is now assigned to a sensibly later period 

 than previously reported. Consequently and however antinomic it may seem, 

 determination must be considered only as a relative concept. Nevertheless, the 

 fact remains, and is duly established by numerous experiments, that in the normal 

 inner embryonic milieu, or in equivalent culture conditions, intrinsic morphochoresis 

 necessarily leads to a decisive change, which suppresses the capacity for regulation. 



Finally, it is important to state that (relative) determination must be considered 

 at the supra-cellular level more than in the individual cells. Stability is better 

 for large communities of cells (Grobstein and Zwilling, 1953; Muchmore, 1956). 

 Moreover, again in the case of the amphibian neurula, if the embryo is submitted 

 to cell dissociation by a controlled digestion of intercellular cements, cells of the 

 same strain are able to join together again; these re-associated groups then move 

 over each other until they form a layer-like, nearly normal topographical 

 arrangement (Fig. 12) which at least resembles the normal advanced neurula 

 (Townes and Holtfreter, 1955). According to Steinberg (1958), this amazing 

 property reveals the existence on the cell surface of a lattice of ionised acidic 

 groups having some affinity for Ca and Mg cations. The spacing of such groups 

 would be different according to the layer or tissue, which would permit cells 

 wandering at random to recognise their likes and make reaggregation selective. 

 Such an enticing interpretation suggests, as does also the para-determination 

 by antisera, that organ specificity is primarily linked with surface changes, whose 

 nature could well orientate cytodifferentiation (see also Spiegel, 1954). 



This is nearly the way we can actually conceive determination, which looks, 

 in a certain sense, like the goal of morphochoresis. Let us now examine how the 



^ According to an oral account at the Providence Conference. 



