EMBRYONIC INDUCTORS AND ORGANIZERS 489 



experiments." No indications of lens development in absence of the optic 

 cup have been observed thus far in the urodeles, Pleurodeles (Pasquini, 

 1927), Triton taeniatus, and T. alpestris (O. Mangold, 1931a); and in 

 Amhly stoma results of earlier experiments were negative or only slightly 

 positive (Le Cron, 1907). Another form of experiment hkewise indicates 

 dependence of lens formation on presence of optic cup in Triton. Removal 

 of the head mesoderm in the early neurula usually results in approximation 

 of the eyes to the median plane or in complete cyclopia, and lenses de- 

 velop in normal relations to the optic cups. This experiment is regarded 

 as doubly significant: first, because the condition determining cyclopia 

 occurs at a relatively late stage of development; second, because the 

 cyclopia results from a local mesodermal defect, while in cyclopia pro- 

 duced by inhibiting chemical agents, or occurring in the reconstitutional 

 duplications resulting from ligature in early stages, the whole embryo is 

 affected, and the locus of the presumptive lens region, as well as position 

 of the optic cup, may conceivably be altered. When the optic primordium 

 is reduced in size, either by reconstitution from a part or otherwise, the 

 lens of R. esculenta is not correspondingly reduced (Spemann, 19126), but 

 in other forms for which data are available there is more or less close cor- 

 respondence in size of optic cup and lens.-^^ 



Transplantation experiments bearing on lens formation may consist in 

 (a) transplantation of presumptive lens epidermis to other regions of the 

 body; (b) transplantation of optic vesicle or cup to other regions or ex- 

 plantation with epidermis from other regions; (c) substitution of other 

 epidermis for presumptive lens-forming epidermis. All these experiments 

 have been performed on at least some amphibian species, some of them 

 on several species. 



Transplantation of presumptive lens epidermis of Amblystoma puncta- 

 tum to other head regions in early and late neurula results in lens forma- 

 tion in the new location (Harrison, 1920); but other species, including 

 R. esculenta, have given negative results (Spemann, 1912a). Transplanta- 

 tions of the optic region of the neural plate (optic plate) or the optic 

 vesicle of a later stage to other body regions have been made in various 

 species with induction of lens. The transplanted optic primordium de- 

 velops and differentiates to an advanced stage, and transplanted pieces 

 may reconstitute to optic cups of small size. Substitution of other epi- 



3' Spemann, 1907, igiia, b; von Ubisch, 1923, 1924, 1925c, 1927; Pasquini, 1931. 

 3^ W. H. Lewis, igoja, b; Spemann, 1912(7; Wachs, 1919; von Ubisch, 1924; Filatow, 1925; 

 O. Mangold, 1931a. 



