276 



Embryogenesis: Progressive Differentiation 



tions," just as one would hesitate to apply 

 this term to the secondary formative effects 

 of the archenteron roof upon the bilaterality 

 of the neural system, or to the accumulation 

 of limb blastema cells following amputation 

 or the insertion of a graft. The kinetic re- 

 sponse of cells to certain stimuli in itself 

 does not indicate the presence of inductive 

 stimuli (see inflammatory reactions). Like- 

 wise, to extend the term "induction" to the 

 effects of external factors upon the rate of 

 tissue growth (as opposed to cell deter- 

 mination) would result in a confusion in 

 terminology. 



There is no doubt that in the course of 

 development new kinds of specific inductors 

 come into play. They may involve the de- 

 rivatives of all three germ layers, and these 

 primordia may cooperate so as to form 

 synergistic '''inductor systems." The differ- 

 ent tissue components of such a system may 

 act simultaneously or in succession but they 

 must be arranged in a typical pattern in 

 order to induce a single structure, such as 

 a typical neural tvibe or an acoustic organ. 

 At this point, we should remind the reader 

 that the outcome of an induction is not only 

 determined by the specific properties of 

 the different inductors but just as much by 

 those of the stimulated cells. The reaction 

 potencies of the latter were found to be lim- 

 ited by the following inherent factors: ge- 

 netic potentialities, stage-specificity, and 

 tissue-specificity. The two independent sets 

 of properties, namely those of the inductors 

 and those of the reacting cells, must be inter- 

 locked in space and time in order to insure 

 normal development. This is principally 

 achieved by directed cellular mass-move- 

 ments. 



COMMENTS ON THE TRANSMISSION 

 OF INDUCTIVE STIMULI 



Although we can no longer doubt that 

 induction involves some sort of an activating 

 chemical mechanism, we really do not know 

 the chemical nature and the mode of trans- 

 mission of these stimuli. 



Schmitt ('41) drew attention to the pos- 

 sibility that the columnar shape of the 

 neural plate cells might result from a mo- 

 lecular zipper effect operating by means 

 of dehydrating agents between the adjoining 

 surfaces of these cells. This interpretation 

 has become improbable in view of the fact 

 mentioned above that isolated medullary cells 

 can elongate reversibly without any surface 

 of contact. But Schmitt applied considerations 



of molecular surface interactions also to the 

 problem of the inductive mechanism, and 

 other workers have speculated along similar 

 lines. 



On the basis of physicochemical models, 

 Needham ('42, p. 289) suggested that in- 

 duction may not actually involve the trans- 

 mission of a chemical agent but may be pri- 

 marily a matter of polar orientation and 

 attraction of long-chain molecides at the 

 inductor-facing side of the reacting cell; 

 this effect might reach into the deeper zones 

 of the cell, creating an increasing complexity 

 of structuration which would lead to neural 

 differentiation. Holtfreter ('44b) also believed 

 that interfacial attraction forces and surface 

 adsorptions might be instrumental in the 

 induction process. Weiss ('47, '49a, b, '50) 

 has elaborated and expanded the concept 

 of intercellular surface actions, making it 

 part of a general hypothesis on the role of 

 "molecular ecology" in morphogenesis. We 

 cannot discuss here the full backgrovmd and 

 the ramifications of this highly interesting 

 hypothesis but shall deal only with its ap- 

 plication to induction. 



The concept of "molecular ecology" is 

 stated by the author as follows: "Each cell 

 and organized cell part (nucleus, chromo- 

 some, etc.) consists of an array of molecular 

 species whose densities, distribution, ar- 

 rangement and groupings are determined by 

 their mutual dependencies and interactions 

 as well as by the physical conditions of the 

 space they occupy. These species range from 

 the elementary inorganic compounds to the 

 most complex 'key' species characteristic of 

 a given cell. Chemical segregation and lo- 

 calization within the cell result from free 

 molecular interplay, as only groups of ele- 

 ments compatible with one another and with 

 their environment can form durable unions. 

 . . . Among the principal segregative factors 

 of molecular mixtures are interfaces. Inter- 

 facial forces between partly immiscible 

 molecular populations concentrate certain se- 

 lected molecular species of the interior along 

 the border. By their surface positions, these 

 border species acquire power over the further 

 behavior of the enclosed system. . . . Various 

 considerations suggest that in biological sys- 

 tems fixation of a given molecular species in 

 a surface is not due solely to unspecific 

 factors, such as surface tensions, adsorption, 

 etc., but that in addition, highly selective 

 chemical affinities are involved. These may 

 be based on the steric interlocking of char- 

 acteristically shaped end groups of the sur- 

 face molecules of adjacent systems" (Weiss, 

 '49b, p. 476). 



