Warren Harmon Lewis 



513 



days after the operation the tadpole was killed. Fig. 10 shows a condi- 

 tion which did not occur in any others of this series. The oedema which 

 commonly occurs in the grafted-on pieces of E. sylvatica seems to have 

 spread to the E. palustris, and has given rise to the thick layer of 

 mesenchyme between the left eye and the skin. This layer of mesen- 

 chyme has prevented the optic vesicle from touching the skin, and as a 

 result the lens has not formed on this side as it did in all the other 

 operations of this series. The right eye, over which the graft was placed, 

 projects towards the coelom of the E. sylvatica. A lens is wanting on 

 this side also, owing, I think, to the lack of contact between optic vesicle 

 and skin. 



oJU^^t^^^^^'-o 



^'"^y^^^v-tJC^jCA^^ 



V\(i. 10. Experiment XI70. Section through the eye reg-ion. The two portions, one on 

 either side of the line ah are drawn from slightly different levels in order to have the 

 section pass through the middle of each optic cup. R. palustris on the right and R. 

 sylvatica on the left. X 30 diameters. 



CONCLUSION, 



It is evident then that the epithelial cells which normally give rise 

 to a lens do not do so when the optic vesicle fails to come in contact 

 with them for a sufficient length of time or at the proper time. On the 

 other hand, when the optic vesicle, entire or regenerated, comes in con- 

 tact at the proper stage of development with the skin on the side of the 

 head a lens generally is formed. So we must conclude, I think, that 

 during normal development the lens is dependent for its origin upon 

 the influence exerted by the optic vesicle upon the skin at the side of the 

 head. This conclusion receives additional support from the experi- 

 ments of section B of this paper in which the optic vesicle stimulated 

 lens formation from skin which under normal conditions never gives rise 

 to a lens. 



