II KINETIC AND CAUSATIVE ASPECTS 35I 



of the organs. It decreases the inductive ability of the prechordal plate {cf. p. 372) 

 and causes presumptive notochordal material to become somitic, and somitic 

 to become pronephritic. The most cephalic parts of these primordia are affected 

 first. The picture of these hypomorphoses is remarkably coherent. They show all 

 grades of what can be called a lowering of the morphogenetic (or morphochoretic) 

 potential, in both cephalo-caudal and dorso-ventral directions. An extensive 

 biochemical study (Lallier, 1954) has largely elucidated the effects of Li"^; it 

 affects mainly the utilization of glucides and the synthesis of proteins. If the 

 suggestion expressed supra has some value, Li"^ would depress the syntheses to 

 the highest grade, according to the duration and strength of its action. NaCNS 

 enhances the formation of chordal material {cf. Corti, 1950), but its antagonism 

 to LiCl does not seem to be as precise as in the sea-urchin. However, the intimate 

 action of certain chemicals on cell metabolism and the physical properties of 

 cytoplasm may be suspected to be of the same nature and probably concern 

 proteins {cf. Ranzi and Citterio, 1954). 



There is another remarkable parallelism in the fact that in vertebrate eggs, 

 effects similar to these of hypermorphic and hypomorphic chemicals may be 

 attained by appropriate operations. These results of microsurgery have in fact 

 been gathered as a consequence of the discovery of the organizer, and mostly 

 from investigations concerned with neurogenic induction. This problem is reserved 

 for our last subchapter, but, assuming that our readers are acquainted with the 

 general trends of modern embryology, it will do no harm to go on with the 

 experimental modifications of the inductive substratum. This will lead us to an 

 immediate consideration of the inconspicuous but important variety of induction 

 which occurs inside the middle layer (see also p. 467). 



{g) Intradermic inductions 



The reciprocal influence exerted in sea-urchin morulae by blastomeres tiers on their 

 neighbour cells could be accounted by an induction process (p. 350) . But it is mainly 

 in the middle layer of amphibian gastrulae, neurulae and young embryos that such 

 processes of induction between parts of the same layer, i.e. intradermic induction, 

 have been recognized. Their prospection has so far not been as exhaustive as for in- 

 terdermic induction. Nevertheless, their interest is considerable, and their thorough 

 analysis could well be necessary to discover the key to the essence of induction. 



When working on a blastula or young gastrida of an amphibian, it is much 

 easier to demonstrate the mutability of the marginal areas than their relative 

 stability. Most operations on the dorsal region show two kinds of effects. When 

 the lowest (presumptively, the most cephalic) part of the notochordal area is 

 extirpated, it is easily replaced in all its capacities by somitic material which 

 moves inwards to close the wound. If the extirpated piece is grafted on the ventral 

 marginal zone of another gastrula (fundamental organizer experiment) two sets 

 of events may occur. {l) In the first, but less known possibility, the graft is too small, 

 or has been a little damaged, or comes from a slightly deficient egg. Its morpho- 

 choretic potential is not maintained, and the graft sinks deeply into the marginal 

 zone: it may form some somites and pronephros, with an induced spinal chord, 

 or, more often, it may migrate along the lateral plates of the host (unelucidated 



Literature p. 483 



