EMBRYONIC RECONSTITUTIONS 523 



depends on level of metabolic activity, that up to the gastrula stage all 

 parts of the blastoderm have a sufficiently high level to reconstitute axial 

 embryonic organs, but this gradually decreases from the beginning of 

 gastrulation. There is, however, a gradient of decreasing activity laterally 

 and around the blastoderm from the region of the longitudinal axis. In 

 earher stages lateral extraembryonic parts are active enough to replace 

 median embryonic parts, but this capacity is gradually limited to the em- 

 bryonic quadrant and finally to the organ regions. In connection with this 

 interpretation the possibihty may also be noted that extraembryonic re- 

 gions may undergo activation when isolated, or by induction when im- 

 planted in contact with higher gradient-levels. The data on differential 

 susceptibility are in line with Luther's interpretation in that they show 

 that the embryonic quadrant attains a higher susceptibility than other 

 regions in early stages. 



Certain experimental data suggest presence of a differential and some 

 degree of dominance, even before formation of the embryonic shield. Un- 

 der inhibiting conditions more than one embryo may develop from the 

 teleost blastoderm." Evidently reconstitution of an embryonic region, 

 inductor, and axis quite independently of the original is possible in tele- 

 osts, and the fact that normally only one embryo forms suggests that the 

 presumptive embryonic region or sector attains some degree of dominance 

 over other parts. However, as far as developmental capacities are con- 

 cerned, the regional differences in early stages are apparently nonspecific 

 quantitative differences in activity in a gradient pattern. 



Pieces of Fundulus germ ring 90° and 180° from the dorsal lip of gas- 

 trulae at any stage, when implanted in extraembryonic regions or in the 

 pericardium, form only epidermis, blood, and chromatophores but, when 

 implanted in the embryonic shield, may develop almost any embryonic 

 part, according to position, or sometimes parts not in accord either with 

 position or with presumptive fate. Some influence, a dominance of some 

 sort by the embryonic shield, is necessary for differentiation of these pieces 

 of germ ring into definite embryonic organs. No evidence of gradation of 

 developmental potencies around the germ ring has been found in this ma- 

 terial (Oppenheimer, 1938). 



CERTAIN RECONSTITUTIONS OF EARLY AMPHIBIAN DEVELOPMENT 



When one blastomere of the two-cell stage is killed or injured so that it 

 cannot develop, but is left in position, the other may develop as a half- 

 embryo (Roux, 1888; A. Brachet, 1927); but, according to Roux, the half- 



" Stockard, 1921; Hinrichs, 1925; see also F. Schmitt, 1901, 1902. 



