Amphibians 



275 



film subject to various states of hydration. 

 He assumed furthermore that induction and 

 cytological determination in general involve 

 the elaboration of new tissue-specific com- 

 poimds, some of which become integrated 

 localized elements of the cell membrane and 

 are thus responsible for the different new 

 types of cellular shape, motility, selective ad- 

 hesiveness, and metabolism. The problem of 

 how, and in which parts of the cell, these 

 specific morphogenetic changes originate is 

 a matter of conjecture (see discussion be- 

 low). 



GENERAL CONSIDERATIONS 



Before entering into a discussion of some 

 general notions on amphibian embryogene- 

 sis, we should point out that our preceding 

 presentation is somewhat unbalanced: much 

 emphasis has been placed upon the phe- 

 nomenon of induction, but other, equally 

 important principles of organogenesis have 

 not received their due share of consideration. 

 We did stress the importance of morpho- 

 genetic movements and of self-organization, 

 but we have neglected such aspects as selec- 

 tive cell adhesion; structural, mechanical, 

 hydrostatic factors of development; space- 

 time patterns of physiological processes and 

 identified substances; problems of energy 

 metabolism; phenomena of differential 

 growth; genetic and comparative-embryolog- 

 ical considerations. For information on these 

 latter aspects, which are in many ways 

 interconnected with the problems dealt with 

 above, the reader is referred to other chap- 

 ters of this book. 



In the following discussion ovir attention 

 shall be mainly focussed upon the concepts 

 of embryonic fields and gradients which 

 have played a predominant role in the analy- 

 sis of induction, and of organogenesis in 

 general. 



GENERAL CHARACTERISTICS OF 

 INDUCTION PHENOMENA 



Since the term "induction" has sometimes 

 been used in a loose way, let us try to cir- 

 cumscribe it for our present purposes. Unlike 

 hormones, inductive stimuli operate only at 

 certain stages, as a rule, during early de- 

 velopment, and they are normally ineffective 

 unless there is an intimate contact between 

 inducing and reacting tissues. The effects of 

 the inductive tissues are undeniable, since 

 in their absence none of the ectodermal and 

 probably few of the mesodermal differentia- 

 tions would ever arise. Once stimulated, the 



cells proceed along their new course of dif- 

 ferentiation independently of a continued 

 application of the inducing stimulus. The 

 newly acquired characteristics are self-main- 

 taining and handed on to subsequent cell 

 generations. In this respect, too, the induc- 

 tive stimuli differ from hormones which 

 must be applied continuously in order to 

 sustain the differentiations initiated by them 

 (for further discussion of these and related 

 problems, see Medawar, '47). 



Thus, normally, "inductors" are living and 

 as a rule embryonic tissues which deter- 

 mine the cytological fate of the reacting, 

 adjacent cells. This inauguration of a new 

 trend of differentiation is almost invariably 

 associated with new trends of kinetic activi- 

 ties of the induced cells, such as invagina- 

 tions, delaminations, new rates of cell divi- 

 sion, etc. But kinetic activities may arise 

 independently of inductive stimuli, as for 

 instance when the blastoporal rim invagi- 

 nates into the blastocoele, or when certain 

 cells establish an epithelium in response to 

 a favorable non-specific substratum. In the 

 same sense, the transformation of cuboidal 

 into temporarily elongated cells, which is 

 practically always associated with ectoder- 

 mal inductions (medullary plate, lens and 

 other placodes: Ruffini, '25; Lehmann, '29; 

 McKeehan, '51), merely indicates that these 

 cells proceed to invaginate. The invaginating 

 cells of the blastoporal lip likewise stretch 

 into long bodies; their change of shape is 

 not due to an inducing svibstratum but to 

 an inherent migration tendency of these cells 

 (Holtfreter, '44a). Columnar cell elonga- 

 tions also appear frequently in intestinal 

 epithelia and their glandular evaginations. 

 Therefore, we cannot subscribe to the idea 

 that there is necessarily a causal relation- 

 ship between cell elongation and induction 

 mediated by attracting and orienting mo- 

 lecular forces acting across the inductive 

 contact surface (Weiss, '47, '50). 



Hence, when speaking of induction, em- 

 phasis should be placed upon the "material" 

 and irreversible rather than upon the "dy- 

 namic" and temporary cellular changes 

 (Vogt, '23-24) following this kind of stimu- 

 lation. In the terms of Weiss ('39), the latter 

 are merely "modulations," that is, reversible 

 manifestations of one among a variety of 

 possible cellular adaptations, the range of 

 which is determined genetically and often 

 with the cooperation of inductors. Therefore, 

 we are reluctant to call the epithelial per- 

 forations produced by contact action in the 

 mouth, pharynx and anus regions "indue- 



