284 



Embryogenesis: Progressive Differentiation 



movement of certain primordia and the out- 

 ward movement and epibolic spreading of 

 others can hardly be explained without in- 

 voking chemical gradients of some sort which 

 control the direction of the cellular migra- 

 tions (Holtfreter, '44a). 



In a stimulating article dealing with the 

 individuation of slime molds from the aggre- 

 gation of equipotential myxamebae, Cohen 

 ('42) points out that in response to the ex- 

 ternal inorganic medium, an initially homo- 

 geneous cell aggregate most likely elaborates 

 concentric patterns of distribution and con- 

 centration of different substances. Critical 

 levels of pH, oxygen tension, salts and other 

 diffusible compounds will be established in 

 certain regions; in this way, the necessaiy 

 conditions are provided for chemical proc- 

 esses that cannot occur in other regions. 

 From these reactions new patterns of com- 

 plexity will arise which in turn initiate 

 further local structurations. Provided the 

 differentially distributed new compounds are 

 of cell-determinative significance, it is quite 

 conceivable that the initially merely quanti- 

 tative gradations play a fundamental role 

 in the segregation of an originally equi- and 

 pluripotential cell-aggregate into definite 

 tissue patterns. 



We believe that such considerations will 

 be very helpful in the analysis of the factors 

 responsible for the self-organization of the 

 various fields in the embryos of higher organ- 

 isms. Clearly, the location of a prospective 

 field within the organism and the size of its 

 cell population would have a decisive 

 bearing on the establishment and effective- 

 ness of such inside-outside gradients. Further- 

 more, regional differences in the accessibility 

 and composition of the environmental fac- 

 tors would determine the outcome. 



It is important to realize that the struc- 

 turation of a field can be inaugurated both 

 by "unspecific" and "specific," that is, induc- 

 tive, factors. Globular cell-aggregates of a po- 

 tential field which float freely in an "unspec- 

 ific" medium may be expected to elaborate a 

 concentric and radially symmetrical organi- 

 zation; this is exemplified in the multiple 

 brain formations of shock-activated ectoderm 

 explants. Local attachment to a substratum, 

 even if it is inert, would introduce polarity 

 into the system. This is demonstrated not 

 only in the basal-apical polarization of at- 

 tached cell aggregates of sponges, slime 

 molds and hydrozoa (Child, '41), but also 

 in the axial organization of embryonic fields 

 of vertebrates which are exposed to a dif- 

 ferential of external conditions. The neu- 



ralized ectoderm explants develop an apical- 

 basal polarity when attached to glass. Simi- 

 larly, the elaboration of a bilateral symmetry 

 in glass-attached explants of blastoporal ma- 

 terial (Fig. 81) is undoubtedly enhanced 

 by external differentials, since freely float- 

 ing explants of this kind fail to manifest 

 their tendency for axial organization. When 

 inductors of any kind are grafted into a 

 whole embryo their action is more or less 

 unidirectional upon the overlying tissues, 

 which simulates the conditions in normal 

 development. In either case, the induction 

 tends to establish a bilateral symmetry and 

 dorsoventral polarity. But such patterns are 

 very rare when the same inductors are 

 placed into a mantle of isolated ectoderm, 

 thus affecting the latter throughout its cir- 

 cumference; under these topographic con- 

 ditions an irregular multiplicity of struc- 

 tures tends to appear. Obviously, and this is 

 in accord with Child's ideas, the emergence 

 of axial patterns in a prospective field re- 

 quires the application of external factors in 

 certain directions. This seems to be true not 

 only of "unspeciflc" environmental condi- 

 tions but still more so of inductive stimuli. 



CYTOPLASMIC FACTORS OF DIFFER- 

 ENTIATION 



It would be desirable to connect the data 

 and concepts derived from the study of am- 

 phibians with those obtained in other fields 

 of biology so as to arrive at a generalizing 

 hypothesis of the factors that determine 

 cellular differentiation. This enterprise is, 

 however, too involved to be tackled satis- 

 factorily within the limited space available. 

 We propose, nevertheless, to venture a few 

 steps into this little explored yet very stimu- 

 lating field of speculation. 



There seems to be agreement among em- 

 bryologists that the regionally different fate 

 of the parts of an egg, and of its subsequent 

 embryonic fields, is determined by cyto- 

 plasmic rather than nuclear differences of 

 the cells concerned. One cannot doubt that 

 the nuclear genes control the emergence and 

 maintenance of the cytoplasmic differentia- 

 tions, yet there is an impressive body of 

 evidence indicating that once local cyto- 

 plasmic differences are established, they may 

 for long periods become relatively inde- 

 pendent of continued gene control. (See, for 

 instance, Hadorn, '36; Sonneborn, '47; 

 Rhoades, '49.) This has led to the supposi- 

 tion that, apart from self-reproducing genes, 

 there are cytoplasmic entities, designated 



