Eye 



405 



cerning the degree of lens dependence in 

 different species is further illustrated by the 

 results of Harrison on Ambly stoma puncta- 

 tum. Removal of the retinal rudiment in the 

 medullary plate stage or of the vesicle 

 shortly after closure of the neural folds re- 

 sults in failure of lens development. Sur- 

 prisingly, however, the presumptive lens 

 ectoderm will differentiate into a lens when 

 grafted to other regions of the head, pro- 

 vided the transplantation is made after clos- 

 ure of the mediillary folds. From this dif- 

 ference in results with a single species "it 

 is apparent that . . . secondary circumstances 

 of some unknown character may dominate 

 more fvindamental ones and thus lead to 

 mistaken conclusions" (Harrison, '20). 



In spite of these seemingly conflicting re- 

 sults, however, the development of the lens 

 continues to provide one of the clearest and 

 most classic examples of dependent differ- 

 entiation. In most species the optic vesicle 

 appears to be the agent primarily responsi- 

 ble for initiating its development, but even 

 in forms such as R. esculenta, where this 

 generalization has not seemed to apply, 

 recent work has shown there is no basis for 

 assuming that the lens is capable of de- 

 veloping independently of inductive influ- 

 ences. Perri ('34) has shown that presump- 

 tive lens epidermis of R. esculenta will not 

 yield lenses when isolated in vitro. Mangold 

 ('31) called attention to the possibility that 

 the roof of the archenteron, which ordi- 

 narily evokes lens formation only indi- 

 rectly, through prior induction of the retina, 

 may instead act directly upon the lens 

 epidermis in such species as R. esculenta (see 

 also Needham, '42, p. 294). New and 

 strongly suggestive clues are provided by 

 the recent work of Liedke ('51) on A. 

 punctatum. In this form, in which the optic 

 vesicle is clearly involved in lens induction, 

 Liedke has shown that the head mesoderm 

 must first prepare or activate the lens epi- 

 dermis during neurula stages before it be- 

 comes competent to respond to the retinal 

 stimulus. There is a close parallel in the 

 development of the ear (Harrison, '45; 

 Yntema, '50), where both mesodermal and 

 neural tissues participate in labyrinth in- 

 duction. The results of Ten Gate as reported 

 by Woerdeman ('50) also help resolve the 

 seeming paradox presented by such forms 

 as R. esculenta. Woerdeman had earlier ('39) 

 been unable to confirm Spemann's results 

 on independent lens formation in R. escu- 

 lenta, and proposed that this might be at- 

 tributable to the difference in temperature 



at which he and Spemann commonly main- 

 tained their embryonic material prior to 

 experimentation. At Woerdeman s instiga- 

 tion i en Gate compared the effects of retinal 

 ablation at the neuruia stage on emoryos 

 previously reared at 10° G. and at 25° G. 

 in the latter series lens formation was very 

 rare, while in the cold-treated group lenses 

 of varying perfection developed in almost 

 all cases. In an independent study, Ten 

 Gate showed that the progress of respiratory 

 and related physiological changes during 

 amphibian development is less retarded by 

 low temperature than is the pace of morpho- 

 logical differentiation; from this it may be 

 presumed to follow that at a given stage of 

 development the tissues (e.g., the epidermis) 

 of a cold-treated embryo will have attained 

 a more advanced level of intrinsic chemical 

 differentiation or maturation than those of 

 an embryo reared at ordinary laboratory 

 temperatures. Ten Gate found that this dis- 

 sociability of morphological and physio- 

 logical differentiation at low temperatures 

 is particularly marked in R. esculenta. 

 These considerations, together with the strik- 

 ing effects of temperature on lens depend- 

 ence, and also the finding of Liedke concern- 

 ing the role of head mesoderm, seem to point 

 to the following picture of lens induction 

 in Amphibia: Probably in all species lens 

 formation is ordinarily elicited by the action 

 of at least two supplementary or mutually 

 reinforcing inductors,* the head mesoderm 

 and the retinal rudiment. Under average 

 environmental circumstances and in most 

 species the component events of development 

 are geared with one another in such manner 

 that the epidermis is insufficiently differenti- 

 ated at the neurula stage to be fully re- 

 ceptive to the action of the head mesoderm, 

 and requires a subsequent and final impetus 

 from the optic vesicle. At low temperatures, 

 however, and especially in species like R. 

 esculenta in which such treatment causes 

 morphological development to lag substan- 

 tially behind chemical differentiation, the 

 lens epidermis has already become suffi- 

 ciently mature and responsive at neurulation 

 to require no further incitement beyond that 

 provided by the head mesoderm. 



Dependent development of the lens has 

 also been clearly established for the chick. 

 Lens formation fails when contact with 

 the optic vesicle is blocked, and transplanted 



* Indications have been encountered that other 

 neighboring elements, notably the nasal placode, 

 may be capable of eliciting lens formation (e.g., 

 Holtfreter, '35; Ikeda, '38). 



