240 



Embryogenesis: Progressive Differentiation 



field the capacity of a segregative "self- 

 organization." 



The following experiment shows that a 

 patterning of this field into separate tissues 

 may occur even in explants in which the 

 original cell arrangement has been changed 

 completely. When the entire dorsal regions 

 of several gastrulae are isolated, then dis- 

 sociated into free cells by means of alkali, 

 and this cell suspension is made to re-aggre- 

 gate in a neutral medium, the composite 

 body will undergo a sorting out of the inter- 

 mingled cells (Holtfreter, '44c). The ento- 

 dermal and ectodermal cells will preferably 

 move into surface positions, while all meso- 

 dermal cells accumulate in the interior. The 

 latter, instead of differentiating into a chaotic 

 mosaic of different cell types, tend to form 

 well segregated tissues, such as somites, pro- 

 nephros, body cavities, and a single, thovigh 

 dendritic, notochord. This self-organization 

 of the disarranged material into a complex 

 and highly integrated structure recalls the re- 

 organization observed in dissociated sponges. 

 It demonstrates that the process of blocking 

 out and determination of the dorsal blasto- 

 poral material into distinct tissue primordia 

 is controlled by several morphogenetic prin- 

 ciples: directed cell movements, selective cel- 

 lular adhesions, and mutual determinative 

 processes occurring between the cellular con- 

 stituents of the aggregate. 



The Ventral Marginal Zone. A determina- 

 tive interdependence did not become ap- 

 parent in explants containing the prospective 

 areas of pronephros, erythrocytes, smooth 

 intestinal muscle and coelomic walls. These 

 tissues developed from the lateral and ventral 

 sectors of the marginal zone, usually com- 

 bined with some epidermis and gut epithe- 

 lium (see also Fernald, '43; Yamada, '50a). 

 Except for the pronephric duct which could 

 appear in any of the lateroventral fragments, 

 the last-mentioned differentiations corre- 

 sponded largely to the prospective signifi- 

 cance of the isolated material as shown on the 

 fate map. However, the conclusion which is 

 suggested by these results, namely that the 

 ventrolateral sectors of the marginal zone are 

 more strictly determined than is the dorsal 

 sector, has become questionable on the basis 

 of subsequent experiments. 



The Entoderm. In contrast to the ectoderm 

 and chorda-mesoderm, the entodermal organ 

 areas appear to be locally determined at 

 the early gastrula stage, although some over- 

 lapping between them may exist. Explants 

 from the different entodermal areas (vegetal 

 hemisphere) furnished the differentiations of 



the various epithelia of the intestinal tract, 

 and of liver and pancreas. These structures 

 could develop independently of each other 

 and in the absence of ectoderm and organizer 

 material. Histological regulations could not 

 be detected, an observation which was made 

 on neurula material as well (Kemp, '46), al- 

 though Balinsky ('47) reports an inter- 

 changeability of the anlagen of liver and 

 stomach at the neurula stage. In order to 

 obtain the epithelial arrangement of an in- 

 testinal wall, the entoderm cells require a 

 support of connective tissue; in its absence 

 they behave like isolated ectoderm forming 

 an irregular mass of cells which, however, 

 becomes cytologically differentiated into in- 

 testinal cells (Holtfreter, '38b,c, '39a,b). Such 

 cell-orienting influences of the environment 

 should not be considered as induction phe- 

 nomena. 



General Conclusions. The differences in 

 developmental capacities of the regions dealt 

 with above indicate that the early amphibian 

 embryo exhibits simultaneously the proper- 

 ties of a detei'minative and a regulative type 

 of development. The entodermal differentia- 

 tions show the highest degree of independ- 

 ence, the ectoderm the lowest, and the chorda- 

 mesoderm occupies an intermediate position. 

 It is, therefore, no longer correct to consider 

 the chorda-mesoderm field as a region whose 

 histological determination precedes that of all 

 other regions, and to relate this alleged 

 supremacy to its faculty of determining the 

 fate of other, less specifically determined, 

 parts of the embryo. Actually, the entodermal 

 districts, being earlier determined than the 

 dorsal mesoderm, are not subject to the 

 determinative influence of the latter. 



Considering, however, the organological 

 rather than the cytological features of the 

 explants, it is evident that none of the pri- 

 mordia can attain the shape and structure of 

 a typical tissue or organ without the forma- 

 tive assistance of surrounding tissues. In the 

 absence of an epidermal or mesodermal cov- 

 ering, all intestinal primordia develop in an 

 inverted orientation forming an external 

 epithelium instead of inner tubes. Without 

 a supporting matrix, especially of mesen- 

 chyme, no ento- or mesodermal epithelia or 

 tubes can be established, the eye vesicle can- 

 not become an eye cup, and no caudal elonga- 

 tion of the axial mesoderm can take place 

 (Holtfreter, '39a,b,c). These environmental 

 factors are mainly of a general mechanical 

 nature and should therefore not be confused 

 with the primary determining factors of 

 differentiation. All evidence points to the 



