282 



Embryogenesis: Progressive Differentiation 



be a determining factor in its future organ 

 pattern. 



Other observations tie up with this notion. 

 Lopashov ('35a) states that the differentia- 

 tions prodviced by explants from the dorsal 

 blastoporal region increase in complexity 

 with the number of fused identical explants. 

 It is well known, furthermore, not only that 

 if the cephalic entomesoderm is progressively 

 reduced in mass it induces increasingly 

 smaller head organs, but also that there are 

 lower limits beyond which well proportioned 

 and complete head structures can no longer 

 be formed. Instead of decreasing proportion- 

 ally in all its parts, the tissue pattern of the 

 reduced heads changes qualitatively. Thus, 

 in the graded series of micro- to anencephalic 

 animals, the lateral structures, such as gills, 

 balancers, eyes, or ear vesicles, become 

 shifted mediad, then appear as single organs, 

 and finally drop out entirely. But even in 

 the complete absence of eyes, the dien- 

 cephalon may still exhibit a rather normal 

 structure (Nieuwkoop, '47). These reces- 

 sions in a lateromedian direction are invari- 

 ably associated with a stepwise reduction 

 and final absence of all head structures in a 

 cephalocaudal direction. 



Lehmann ('45, '48) circumscribes these 

 changes of pattern in terms of "Realisations- 

 stufen" and he points out that corresponding 

 situations exist in other blastemas, such as 

 those of the ear vesicle (Andres, '48) or the 

 limb (Bretscher, '49). As a matter of fact, this 

 relationship between initial size of the field 

 and the diversity of its final differentiations 

 has been found previously in many other in- 

 stances, especially in invertebrates, where 

 this phenomenon was described under head- 

 ings such as vegetative budding, regenera- 

 tion, or reconstitution of disarranged organis- 

 mic fields (for references see Child, '41; 

 Huxley and De Beer, '34; Berrill, '41; Holt- 

 freter, '51). In all these instances, the initial 

 size of the field seemed to determine its fu- 

 ture organological complexity, each field 

 having its own series of critical size thresh- 

 olds which determine type and configura- 

 tion of the tissues that will emerge from the 

 originally pluripotential mass of cells. 



AMPHIBIAN DEVELOPMENT AS 

 RELATED TO CHEMICAL GRADIENTS 



Child's Gradient Theory, according to which 

 structural patterns are preceded and deter- 

 mined by axial gradients of metabolic activ- 

 ity, is well known and need not be elaborated 

 here. This theory, which has its main and 



legitimate field of application in reconstitu- 

 tion processes of invertebrates, has also been 

 applied to amphibian development (general 

 references: Huxley and De Beer, '34; Child, 

 '41, '46). From what follows it appears, how- 

 ever, that the factual basis for this latter 

 application is rather slim. 



Let us first acknowledge that not all em- 

 bryonic fields originate throvigh induction. 

 In an aggregation of a sufEiciently large 

 number of amebocytes of Dictyostelium 

 (Raper, '41), a field arises "autonomously," 

 that is, under "unspecific" external condi- 

 tions. A corresponding self-establishment of a 

 field appears to occur in an isolated piece 

 from any region of the gastrula ectoderm of 

 anurans which tends to segregate into epi- 

 dermal and sucker cells (Holtfreter, '33a, 

 '36, '38c; Yamada, '38; Raunich, '42a). As 

 in the classical experiments on hydrozoans 

 and worms, the mere act of "physiological 

 isolation" from controlling factors of the 

 whole organism (Child, '15, '41) seems to 

 create the new field. 



No other examples of this sort are known 

 in amphibian development, and it has been 

 mentioned already that no new fields could 

 be produced simply by raising the rate of 

 metabolism of an embryonic district. The 

 production of a brain-field in ectoderm ex- 

 plants by means of injurious treatments cer- 

 tainly resembles the individuations obtained 

 by Child ('41) in injured hydrozoa, but it 

 is doubtful that Child's rather vague con- 

 cepts of differential susceptibility, recovery 

 and dominance are of much help for a con- 

 crete physiological interpretation of the above 

 results. 



Principally on the basis of the data of 

 Bellamy ('19) on the regionally different 

 susceptibility of amphibian gastrulae to the 

 cell-dispersing action of potassium cyanide, 

 Child postulated a region of "physiological 

 dominance" in the ectoderm around the ani- 

 mal pole, and a dorsoventral gradient with 

 its center above the dorsal blastoporal lip. 

 The differential susceptibility to potassium 

 cyanide was interpreted as indicating dif- 

 ferences of oxygen consumption. However, 

 later workers refuted this interpretation 

 (Buchanan, '29; Holtfreter, '43a). Subse- 

 quent quantitative determinations did reveal 

 regional differences, althovigh not exactly 

 gradients, in the distribution of certain com- 

 pounds, or physiological processes, within 

 the early amphibian embryo. This was shown 

 in the case of sulfhydryl compounds, RNA, 

 alkaline phosphatase, degree of reducing 

 power, glycolytic activity, and rate of oxy- 



