EMBRYONIC RECONSTITUTIONS 



527 



connection the eye is of particular interest. In the early gastrula of Triton 

 the whole presumptive epidermis can give rise to eyes, under influence of 

 the dorsal inductor; but eye potency decreases from apical to basal regions, 

 that is, down the primary gradient.'^' After the neurula stage only the 

 optic primordium gives rise to eye, but the capacity of the optic vesicle 

 and optic cup for reconstitution of a whole eye from parts homoplastically 

 transplanted to other embryonic regions supposedly without eye-deter- 

 mining factors — for example, the ear region — is great. Any part can recon- 

 stitute any part: even small pieces from any part of the tapetum can be- 

 come whole eyes. In general, capacity for organization increases with size 

 of the transplanted piece. The larger the piece, the more evident is the 

 heteropolar pattern. Extremely small fragments usually give rise only to 

 the pigment layer, but in larger pieces the retina may be massive.'^ The 

 orderly character of development in the transplanted pieces and the rela- 

 tion between organization and size of piece have led Dragomirow to re- 

 gard the optic vesicle as a gradient system in which the retinal region is 

 dominant. The pigment layer represents development from lower gradi- 

 ent-levels. He finds these embryonic optic reconstitutions closely analo- 

 gous to reconstitutions in hydroids and planarians. Pieces of iris of urodele 

 larvae of various ages implanted in the orbit after removal of eye or in the 

 abdominal cavity may reconstitute retina; and in the orbit, nerve libers 

 (Monroy, 1939). 



In marked contrast to the high reconstitutional capacity of the optic 

 vesicle and various other parts in later embryonic stages is the failure of 

 the early tail bud to reconstitute parts removed,'^ although a high capac- 

 ity for regeneration is characteristic of later stages of tail development. 

 Two possible factors of this failure may be pointed out. For reconstitu- 

 tion presence of spatial pattern and reaction to removal of a part are neces- 

 sary : a mass of cells all alike cannot reconstitute anything if part of the 

 mass is removed; also, if level of activity is so high that removal of a part 

 does not bring about activation, there is no reconstitution, and the re- 

 maining part continues as before. The early tail bud is evidently a mass of 

 actively growing cells, perhaps with a slight radial gradient, like other 

 buds, but probably with so little differential and dominance and so high a 

 level of activity that removal of a part leaves the remaining part practi- 



"■ See Mangold, 1931a, for literature. 



'7 Dragomirow, 1932, 1933, 1935, and earlier literature cited in these papers. His experi- 

 ments include three species of Triton, Amblystoma mexkaniim, Pelobates fnsciis, and Bom- 

 binator igneus. 



'* Schaxel, 1922; V'ogt, 1931; Svetlov, 1934. 



