Amphibians 



281 



but which have no control over the organ- 

 ized pattern of these differentiations. 



To conclude, then, the "organizer" has the 

 characteristics of a niorphogenetic field 

 which is, however, not really harmonious- 

 equipotential in the strict definition of 

 Driesch, and it induces another, or several 

 other, fields, which are likewise capable of 

 regulation and of self-organization. 



Self-organization of Fields. We have had 

 several occasions to emphasize the important 

 role which the self-organization of mor- 

 phogenetic fields plays in progressive dif- 

 ferentiation: an area which is capable of 

 self-differentiation and of regulation breaks 

 up into smaller units which may represent 

 fields on a smaller scale and with more re- 

 stricted differentiation potencies. They, in 

 turn, may subsequently be subjected to 

 further segregation, until the final organiza- 

 tion of an organ is achieved. One of the 

 outstanding characteristics of the process of 

 "self-organization" is its autonomous char- 

 acter; it is illustrated by the morphogenetic 

 behavior of the chorda-mesoderm field (Fig. 

 81) and the limb field, and by the highly 

 organized differentiations induced by adult 

 organs (Figs. 100, 103, 104) and in xeno- 

 plastic combinations (Fig. 97). 



This issue, and its significance for verte- 

 brate development, has been recognized 

 early by different investigators. Weiss has 

 stressed its importance under the heading 

 of "autonomization" ('26) and "emancipa- 

 tion" ('35, '39); Lillie ('27) has used the 

 term "embryonic segregation" and Lehmann 

 ('42b) the term "autonomovis self-organiza- 

 tion." The earlier cell-lineage studies and 

 isolation experiments on invertebrate eggs 

 had already focussed the attention to this 

 principle. 



In contrast to the extensive analysis to 

 which embryonic induction has been svib- 

 jected in the last 50 years, our information 

 on the mechanisms involved in the process 

 of self-organization is negligible. Tentative 

 approaches to this problem have been made. 

 In connection with his investigations on the 

 development of the limb and otocyst in 

 Amblystoma, Harrison ('45) has suggested 

 that the emergence of axial structuration in 

 a field might be based on the presence of 

 a supracellular paracrystalline lattice of di- 

 polar molecules. Spiegelman and Steinbach 

 ('45) have interpreted the differentiation of 

 morphogenetic fields in terms of physiologi- 

 cal competition between transforming cells, 

 in the framework of a more general and 

 more elaborate theory which we cannot dis- 



cuss in detail. Weiss ('50) points out that 

 neither electrodynamic theories nor simple 

 concentration gradients nor differences in 

 position and exposure of cell groups can ac- 

 count for the complex behavior of segregat- 

 ing fields. He suggests ('50, p. 194) that "It 

 may become necessary to assume that at any 

 given point of a particular field, conditions 

 of such specific constellation arise that cer- 

 tain molecular groupings will be selectively 

 favored or energized by a sort of resonance 

 relation between field and molecular pat- 

 tern." 



The lack of precision and the diversity 

 of these notions testify to oiir present-day 

 ignorance of the physicochemical factors op- 

 erating in the process of field segregation. 

 But the progress which is being made in 

 the elucidation of some instances of embry- 

 onic fields (see below) is encouraging 

 enough to support the belief that the crucial 

 and universal problem of self-organization 

 is not entirely refractory to further analy- 

 sis. 



Morphogenetic Movements as an Organizing 

 Principle. The patterning of an embryonic 

 field involves more mechanisms than deter- 

 minative (inductive?) interactions of its 

 parts. In some instances, implication of the 

 latter is even doubtful. For example, the 

 re-individuation of a disaggregated adult 

 sponge is mainly, if not entirely, mediated 

 by the principles of directed movements and 

 selective adhesion of the different types 

 of cells. Corresponding morphogenetic cell 

 movements are of equal importance for the 

 tissue patterning of the chorda-mesoderm 

 field. That these movements of invagination, 

 spreading or stretching which make for the 

 regroupings and final segregation of the 

 tissue components are inherent properties of 

 the cells concerned is clearly demonstrated 

 in explants from the blastoporal region, 

 especially if such explants have been previ- 

 ovisly disaggregated. 



Embryonic Fields as Related to Their Initial 

 Mass. Graded inactivation by heat of either 

 normal or atypical inductors reduces their 

 effectiveness in a quantitative as well as a 

 qualitative sense. The inductions not only 

 become progressively smaller but they ex- 

 hibit less organized patterns until they have 

 the aspects of non-specific neural or neuroid 

 cell groups. This reduction and final loss of 

 patterning seems to show that the organo- 

 logical complexity of a neiu-ogenic field de- 

 pends primarily on the number of cells 

 which have experienced stimulation. In 

 other words, the size of the field appears to 



