404 



Special Vertebrate Organogenesis 



'43), in connection with his interesting 

 studies on the origin and development of 

 the iris pigment. 



INDUCTION OF THE LENS 



Ahhough it had been proposed earlier that 

 lens formation is influenced by the develop- 

 ing optic cup, Spemann ('01) was the first 

 to test the issue by direct experiment. First 

 by cautery, and later by surgical excision, he 

 removed the rudiment of the retina in the 

 open medullary plate stage of Rana fusca. As 

 a consequence the lens failed to form, and 

 since the results were so unequivocal, it 

 seemed natural at the time to assiime that 

 a similar dependency of the lens would be 

 found to obtain imiversally. However, this 

 conclusion soon proved to be imwarranted. 

 Mencl reported instances of spontaneous lens 

 occurrence in Salmo, and King found that 

 the lens developed in R. palustris following 

 experimental removal of the optic cup. Spe- 

 mann, at first critical of these reports, was 

 nevertheless stimulated to extend his investi- 

 gations to other forms ('12) and, although 

 he verified completely his original results 

 with R. fusca, found that complete removal 

 of the optic rudiment in R. esculenta was fol- 

 lowed by the development of well formed 

 lenses. In Bombinator, although normal 

 lenses never arose in the absence of the op- 

 tic cup, there were nevertheless unmistak- 

 able indications of incipient lens develop- 

 ment. 



Along with these early experiments on 

 the ablation of the optic vesicle, the lens- 

 inducing capacity of the vesicle was fvirther 

 tested by replacing the normal lens-forming 

 epidermis with epidermis from other regions 

 and also by introducing the optic vesicle 

 beneath the epidermis in foreign positions 

 on the embryo. The latter procedure was first 

 employed by W. H. Lewds, who found that 

 in numerous cases lenses formed in R. syl- 

 vatica and R. palustris over the transplanted 

 vesicle. He also obtained lenses w^hen epi- 

 dermis from the trunk was transplanted in 

 substitution of the normal lens-forming epi- 

 dermis. 



Whereas in R. palustris and R. sylvatica 

 epidermis from any portion of the body 

 was capable of giving rise to a lens, Spemann 

 found that in Bombinator only epidermis 

 from the head region had this capacity, 

 and in R. esculenta the lens-forming capacity 

 was restricted to the presumptive lens- 

 forming cells themselves. Spemann called 

 attention to the inverse relationship here 



revealed between the degree of dependency 

 of the lens upon the optic vesicle and the 

 distribution of lens-forming potency within 

 the epidermis. Thus in R. palustris, in which 

 apparently the entire epidermis is capable 

 of forming a lens, the predisposition to do 

 so is at no point strong, whereas in R. escu- 

 lenta, in which the lens-forming potency is 

 limited to the presumptive lens-forming tis- 

 sue itself, it is — in this circvxmscribed re- 

 gion — highly developed.* Bombinator oc- 

 cupies an intermediate position, both with 

 respect to the extent of the area (head ecto- 

 derm) capable of forming a lens, and in the 

 degree of dependency of the lens ectoderm 

 upon stimulation by the vesicle. 



One must of course bear in mind that the 

 distribution of lens-forming competence in 

 the epidermis of these species is largely a 

 function of time, i.e., stage of development. 

 Even in R. esculenta, lens competence is 

 presumably widespread in the ectoderm of 

 the young gastrula, while in species like 

 R. palustris reactivity eventually narrows 

 to the lens placode itself. In R. catesbiana, 

 in which the dependence of the lens is espe- 

 cially pronounced, the competence of trunk 

 ectoderm persists until relatively advanced 

 tail-bud stages (Pasquini, '32). Even after 

 the lens rudiment has been initially deter- 

 mined, or in fact has already begun to show 

 visible thickening, continued association 

 with the optic cup is necessary in order for 

 it to achieve normal structure. If isolated 

 from the cup at these stages by heterotopic 

 transplantation, it gives rise only to abortive 

 lenses. To varying extents, the lens in all 

 other species as well is probably dependent 

 on similar reinforcement for completion of 

 its differentiation and particularly its growth 

 in size (see, for example, LeCron; Woerde- 

 man, '39, '50). 



The sharpness of the distinctions between 

 the degree of dependence of the lens upon 

 the vesicle in the species studied by Spemann 

 was questioned by von Ubisch, whose results 

 indicated that external conditions such as 

 temperature influence the percentages of 

 lenses obtained. An even stronger qualifica- 

 tion of Spemann's results with R. esculenta 

 is provided by the more recent findings of 

 Woerdeman, to be cited shortly. The neces- 

 sity for caution in drawing conclusions con- 



* Although the development of the lens in R. 

 esculenta appears to be independent of the optic 

 vesicle, the latter nevertheless possesses the ability 

 to induce a lens. This was shown by Filatow, who 

 found that the optic vesicle of esculenta grafted be- 

 neath ventral epidermis of Bufo vulgaris was ca- 

 pable of inducing a lens. 



