Amphibians 



277 



The highly specific molecular configura- 

 tions at the cell surface are thought to play 

 a particularly important role in differentia- 

 tion, tissue affinities and embryonic induc- 

 tion. Assuming that the cell surfaces of 

 inductive tissue are characterized by specific, 

 oriented end groups, "the molecular surface 

 film of the inducing layer (e.g., retina) 

 would selectively attract key molecules of 

 complementary or otherwise conforming con- 

 figuration from the interior of the cells of 

 the adjacent layer (e.g., epidermis) and thus 

 decide the further differentiation of these 

 cells" (Weiss, '49b, p. 478). The incorpora- 

 tion of certain key molecules in the svirface 

 would result in the competitive removal of 

 others from the surface, and this redistri- 

 bution wovild initiate a specific trend of 

 differentiation within the reacting cell. The 

 inductor itself wovild operate only on the 

 interface of inducing and reacting tissue by 

 creating on the surface of the latter a "steric 

 conformance, i.e., complementary spatial 

 configurations between molecules or certain 

 exposed atomic groups of them enabling 

 them to conjugate in key-lock fashion" 

 (Weiss, '47, p. 256). 



A diffusion of molecules from the inducing 

 cells to the interior of the reacting cells 

 is not considered as an alternative mechan- 

 ism. Since attempts to test the diffusibility 

 of inductive agents of normal inductors by 

 the vise of separating membranes of known 

 porosity have failed so far (Brachet, '50; 

 McKeehan, '51), the old assumption that 

 under normal conditions close contact be- 

 tween inducing and reacting cells is neces- 

 sary, remains unchallenged.* 



The above concept of Weiss meets with 

 serious difficvxlties when we consider that 

 freely floating ectodermal explants can be 

 neuralized by different liqviid media which 

 have no fixed molecular configuration. This 

 can occur in the absence of any disintegrat- 

 ing cells (Holtfreter, '47b). Conseqviently, 

 there is no likelihood that, as Weiss ('49a) 

 suggests, an "inductive protein film" might 

 become adsorbed at the surface of the float- 

 ing explant. Furthermore, it is difficult to 

 apply this hypothesis to the following ob- 

 servations: transformation of noninductors, 

 such as ectoderm or entoderm, into neural 

 inductors by different killing procedvires; 



* Very recently, Niu and Twitty ('53) demon- 

 strated that the inductive factors of living tissues are 

 indeed diffusible. Ectoderm explants differentiated 

 into neural and mesodermal tissues through the 

 application of the salt solution in which pieces of 

 embryonic inductors had previously been cultured. 



the neuralizing capacity of such diverse sub- 

 strata as the normal archenteron roof, adult 

 organs of all sorts and their cell-free extracts. 

 It can hardly be assumed that all of them 

 possess the same specific molecular surface 

 pattern to account for their common neural- 

 izing effects. 



It appears, therefore, that the kinetic and 

 differentiation tendencies of induced cells 

 can arise independently of a molecule-orient- 

 ing inductor and that the new cell proper- 

 ties are evoked by the introduction of 

 non-oriented chemical agents rather than by 

 the orienting or attracting forces of a spe- 

 cifically structured substratum. A cytotypical 

 molecular configuration of the cell surface 

 which seems to be very important for cell- 

 specific motility, adhesion and differentia- 

 tion, may very well arise secondarily and 

 independently of a contact with other cells. 

 Hence the burden of organizing the cellular 

 constituents for cytodifferentiation would be 

 carried by the reacting rather than by the 

 inducing cells, a relationship similar to that 

 between sperm and egg development or be- 

 tween a hormone and the differentiations 

 initiated by it. We do not question the fact 

 that polarity, shape, arrangement and move- 

 ments of all embryonic and adult cells in- 

 clviding cellular "modulations" (Weiss, '39) 

 are subject to controlling factors of the en- 

 vironment. However, the latter are merely 

 subsidiary and not determinative factors. 

 Their presence enables the cells to manifest 

 their inherent potentialities which have been 

 previously determined by cell-inherent or 

 extraneous factors. 



THE QUESTION OF QUANTITATIVE 



VERSUS QUALITATIVE CHEMICAL 



DIFFERENCES BETWEEN THE 



DIFFERENT INDUCTORS 



Once the chemical nature of inductive 

 processes was established, there arose the 

 question: how does the diversity of induced 

 structures come about? This question is very 

 complex, as may be deduced from the fact 

 that histologically different primordia can 

 induce the same kind of tissue, and that a 

 single primordium can indvice one structure 

 in one germ layer and another one in an- 

 other germ layer. In a simplifying fashion 

 this problem has been reduced to the ques- 

 tion of whether the stimuli of the different 

 indvictors vary in a quantitative or in a 

 qvialitative sense. 



It should be realized that this question 

 cannot be answered satisfactorily by any ex- 



