Amphibians 



283 



gen consvimption, all of which follow more 

 or less the same pattern of distribution (for 

 references see Brachet, '45; Boell, '48). These 

 patterns, however, do not coincide at all with 

 those of the inducing fields or their induc- 

 tions. All they appear to show are regional 

 differences in the rate of metabolic activity 

 of the various germ layers or primordia. 

 These differences seem to be largely a re- 

 flection of tissue-specific differences in the 

 ratio between inert yolk and physiologically 

 active cytoplasmic components. Some parts 

 of the embryo — notably the ectoderm, which 

 is poor in yolk reserves — differentiate com- 

 paratively faster than others, but the bio- 

 chemical data have so far failed to throw 

 any light upon the phenomena of induction, 

 regulation or tissue determination. Even if 

 there were a close parallelism between met- 

 abolically especially active and morphogenet- 

 ically "dominant" regions in the sense of 

 Child, one may still advance the argument 

 of Spemann ('38) and others that it is diffi- 

 cult to decide whether such metabolic pat- 

 terns are the cause or the effect of tissue 

 determination. 



The Double Gradient Theory of Dalcq and 

 Pasteels ('37, '38; Dalcq, '41a) follows similar 

 lines. These authors have postulated that all 

 embryonic tissue differentiations, including 

 the intramesodermal segregations, are cavised 

 by one and the same hypothetical agent ("or- 

 ganisine") and that qualitative differences 

 between the inductions are due to different 

 concentrations of this agent. The premises 

 of this gradient concept and some of its ap- 

 plications to embryological problems have 

 already been discussed in previous chapters. 

 No doubt this concept aims to be all-embrac- 

 ing. It attempts to attribute such diverse 

 phenomena as egg organization, morphoge- 

 netic movements, field segregation, and re- 

 gional induction to the interplay of just two 

 hypothetical factors, a cortical and a vitel- 

 line factor, whose interactions would result 

 in the establishment of the aforementioned 

 gradient of "organisine" that pervades the 

 whole embryo. It is impossible in this re- 

 view to evakiate critically the factual and 

 theoretical aspects of this hypothesis. It is 

 based upon assumptions which seem to be 

 controversial or arbitrary, and some of the 

 interpretations offered are merely circum- 

 scriptions of the problems to be solved. 

 Similar criticisms were raised by Rotmann 

 ('43). We assert again that in most instances 

 when this concept has been applied to cer- 

 tain observations, other hypotheses would 

 serve as well, if not better. It seems that too 



many unrelated, though partly overlapping, 

 processes are engaged in embryogenesis to 

 allow for their unitary interpretation in 

 terms of an oversimplified gradient concept. 



Yamada ('50a, b) has proposed a different 

 version of a double gradient theory. He 

 postulates two qualitatively different activ- 

 ities distributed in a gradient fashion and 

 changing their values with time. One of these 

 "morphogenetic potentials" would be re- 

 lated to the dorsoventral pattern of organiza- 

 tion in ectoderm and mesoderm and have its 

 highest concentration at the dorsal side. The 

 other would be related to stretching and 

 convergence activities; it would be repre- 

 sented by a cephalocaudal gradient with its 

 peak at the caudal end. The specific differen- 

 tiation of a germ area would be determined 

 by the combined effects of both. 



The spatiotemporal patterns of both poten- 

 tials are thovight to be controlled by extrinsic 

 as well as intrinsic factors and therefore 

 modifiable experimentally. For instance, the 

 dorsoventral and cephalocaudal potentials 

 of ventral ectoderm remain low when the 

 ectoderm is isolated, resulting in atypical 

 epidermis. A change of pH results in brain- 

 like differentiations which are interpreted as 

 a raise of the d-v potential ("dorsalization") 

 without a change of the cephalocaudal poten- 

 tial. The same ventral ectoderm, when com- 

 bined with guinea pig kidney, differentiates 

 into tail-like structures including spinal cord 

 and somites, which is considered to be the 

 result of the raising of both potentials ("dor- 

 salization" and "caudalization"). Other ex- 

 periments are interpreted along the same 

 lines. However, the experimental data on nor- 

 mal and atypical inductions can be inter- 

 preted equally well in terms of "neviralizing" 

 and "mesodermizing" inductors, and more 

 factual evidence would be required for a 

 critical evaluation of this hypothesis. 



Inside-Outside Gradients as Determining 

 Factors in the Organization of Embryonic 

 Fields. Concentration gradients involving not 

 just one, but many kinds of chemical sub- 

 stances as well as more complex organic 

 entities are bound to develop in any embry- 

 ological system in response to its environ- 

 ment (Giljbs phenomenon). The concentric 

 organization of an amphibian egg into ovo- 

 plasm, pigmented cortex and coat can be 

 partly attributed to this principle of sorting 

 out of surface-active substances and their 

 subsequent reactions with each other and 

 with external factors. Such inside-outside 

 gradients are significant even in the multi- 

 cellular embryonic stages where the inward 



