11 KINETIC AND CAUSATIVE ASPECTS 353 



and even pulsating cardiomyoblasts. This is a considerable diversification, which 

 has been interpreted as the production of new fields potentially contained in the 

 presumptive somites^ (Muchmore, 1956). 



Another interpretation seems possible. If it is admitted that the fate of a given 

 amount of mesoblast depends on certain products of metabolism, whose concen- 

 tration corresponds to a definite potential (notochordal, somitic, pronephritic, 

 etc.) and that these products, or some of their antecedents, can be transmitted 

 to other cells by difTusion or otherwise, juxtaposition of cell groups belonging to 

 diflferent levels may result in a new equilibrium. At first this new equilibrium 

 expresses itself in a continuous gradient; secondarily, owing to a threshold effect, 

 it takes the form of a scalar distribution, yielding adjacent correlative organs. 

 From the new results of Muchmore, it might be supposed that, at the periphery 

 of the explanted somitic lump, slight lesion and especially dispersion of cells 

 could result in some dilution of the morphogenetic products. This would be the 

 first step towards the secondary production of adjacent fields with their own type 

 of organization. It must be added that at least one striking experiment strongly 

 suggests that the institution of regional fields is linked with the elaboration of 

 definite products inherent to organs such as notochord, neural cord, myotomes, 

 pronephros, which are also transmissible to receptive cells. Reference is made here 

 to the experiment in which lumps of ectoblast of a newt gastrula are superimposed 

 upon several organs of advanced neuruia or young tailbud stages and form chordal, 

 nervous, muscular, pronephritic tissue, according to the organ to which they are 

 juxtaposed or to the field in which they are integrated (Holtfreter, ig33a). If this 

 result really reveals the gradual elaboration of organ-specific substances, it falls 

 well in line with the previous data. 



Qiiite recently, it has been found that the moment of axial organ stabilisation 

 actually occurs somewhat later during neurulation than was believed. If Triturus 

 neurulae, at the stage of a well formed neural plate are exposed during 30 to 

 60 min. to a temperature of 30 to 40'^ C, their development is momentaneously 

 stopped and then recommences, but with a twinning effect. This process is 

 accompanied by a rapid enlargement of the neural plate (Takaya and Watanabe, 

 1957). This is a further encroachment on the idea of sudden determination. 



In this issue as in many others, the difference of opinion revolves around 

 preformation versus epigenesis. The current conception admits that, in the early 

 neuruia, fields of pronephros, limb and heart exist potentially already, broader 

 than the organ will be, and overlapping each other. This concept (Holtfreter, 

 1938b, his Fig. 68) is probably acceptable for stages immediately preceding 

 the appearance of the new organs, but to reflect these ideas back on the young 

 neuruia^ or gastrula may seem inadequate. This interpretation necessarily 

 postulates latent potencies, and forces us to imagine an exceedingly complex 

 organization. However, a simpler solution is possible (p. 474). 



^ For the problem of meristic segmentation, see Dalcq (ig47a), Waddington and Deuchar 

 ^1954)- 



An experimental analysis of determination and differentiation of the mesoblastic struc- 

 tures of the neuruia has been made in Urodeles by Chuang and Pai ( 1 956) . They are opposed 

 to the idea of overlapping fields and favorable to the concept of a general induction field. 



Literature p. 483 



