474 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



become especially useful to explain transformation in the visual organs. Again, in this 

 restricted field, the biochemical substrate of these relations is certainly modified, but any 

 precision in our knowledge is lacking. 



Our point of view also makes intelligible the close relation between myogenic and neuro- 

 genic induction. Except in some exceptional cases discussed p. 445, the same agents which 

 induce the neuroepithelium at the surface evoke mesoblast and even chordo-mesoblast in 

 the deeper layers (p. 445). The conditioned medium of an organizer culture induces myo- 

 blasts as well as neuroblasts (p. 409). Various experiments cause a local chordalization of 

 the neuro-epithelium. This can even happen with a prechordal derivative such as retina 

 (Fig. 74, p. 413), which is a good sign that there is a basic affinity between these apparently 

 distinct differentiations. It is not too audacious a guess to consider that in the archenteron 

 roof, in an implantation- or in a sandwich experiment, much depends on the amount of 

 inducing agent that the (chordo-) mesoblast reserves for its own differentiation in com- 

 parison with the amount which is allowed to escape, apparently after some transformation, 

 and to exert neurogenic induction. The prechordal mesoblast is, so to speak, the least 

 greedy, while the caudal mesoblast even absorbs a supplement of inducing agent (p. 368, 

 note). Experimentally, the balance may be shifted nearly completely either in favor of the 

 spinal cord (Fig. 87, p. 436) or in favor of the chordomesoblast, which is formed without a 

 trace of neuroepithelium (Fig. 91, p. 441). 



A w^ord should be added about the concept of morphogenetic fields. In early 

 stages, the expression of morphogenesis in terms of fields is easy to grasp; it simply 

 designates a certain area characterized by homogeneous but varying properties. 

 At later stages, the concept becomes less clear, for it points to the territory sur- 

 rounding a forming rudiment, e.g. lens, ear, or limb and suggests the existence of 

 ambiguous latent potencies (see p. 353). A solution has been proposed by Kawa- 

 kami (1952) in an interesting discussion of several operations yielding secondary 

 inductions. In his own terms, "mesoderm having strong inductive capacity 

 operates as the inductor of the sensory organs, while the same, with weak inductive 

 capacity, deterrrtines the field of these organs". Other conditions like cell dispersion 

 and competitive relations, could also be considered, but progress toward a logical 

 explanation is sufficiently indicated. 



[e) On the nature of the inducing agents 



As soon as it was recognized that induction was not necessarily restricted to 

 living parts of the organizer, the opinion prevailed that the process was caused 

 by substances released by the inductor, and several workers tried to find out their 

 biochemical nature. They soon realized the complexity of the problem, and the 

 importance the part played by secondary elaborations inside the reacting tissue. 

 However, there was no doubt that a substantial flux was at work, until Weiss ( 1 949a) 

 drew attention to the probable necessity of contact between the protagonist 

 tissues. He suggested that transmission was questionable and could be tentatively 

 replaced by an occurrence of physico-chemical changes due to mutual influences 

 between the cell surfaces causing consequences within the molecular population 

 of the cytoplasm. This proposal deserved and received consideration, but was not 

 so far confirmed. Its merit has been to emphasize the undeniable importance of 

 the surface properties of cells, but the modification of these surfaces without 

 penetration of extraneous molecules seeins entirely theoretical, at least for induc- 

 tion processes. In induction, the available evidence remains favorable to an ab- 



