534 Experimental Studies on the Origin of the Lens in Amphibia 



and to suggest also that there are at this early stage, shortly after the 

 formation of the otic capsules, chemical differences between the epithe- 

 lium of the skin and otic capsule. The same question may be applied 

 to the optic vesicle. Before its contact with the ectoderm and again 

 after invagination and differentiation have progressed, does it possess 

 the power to stimulate lens formation? In a subsequent paper I shall 

 show that the optic vesicle when cut away from the brain at an early 

 period and transplanted to other parts of the embryo retains its power 

 of progressive development, of invagination and differentiation of the 

 layers of the retina. Does it lose with these progressive changes its 

 power to stimulate lens formation? Some of the experiments already 

 cited would seem to show that for a short time at least it retains this 

 power. 



Does the reaction between the optic vesicle and ectoderm require a 

 definite length of time, and will a decrease in this period influence the 

 development of the lens? After the initial stimulus has been given for 

 lens formation does it become self-differentiating or is continued contact 

 with the optic cup necessary for the normal development of the lens? 

 ISTormally the lens remains closely adherent to the optic cup until after 

 the former has separated from the ectoderm and only when the vitreous 

 humor begins to form does it separate from the retina. Is this normal 

 attachment necessary for the growth and differentiation of the lens? 

 Will it cease to grow or will it grow abnormally if its normal relations 

 with the optic cup are altered? A few experiments which I have done 

 would seem to indicate that even as late as the time of separation of the 

 lens from the ectoderm the growth of the lens will be retarded if the optic 

 cup is removed. The results of some experiments by Schaper ' can be 

 explained on the supposition that the reaction between the optic cup 

 and lens is of considerable duration. In Schaper's experiments the lens 

 thickenings were pulled out of the shallow cavity of the optic cup, evi- 

 dently by the dorsal growth of the ectoderm to cover the large wound 

 resulting from removal of most of the central nervous system. In this 

 dorsal shifting of the ectoderm the lens rudiment was also involved. 

 The lens has thus been removed from its normal intimate relation with 

 the retina with the result that the lens has remained in a very rudimen- 

 tary condition and in nearly all of his experiments has not even separated 

 from the ectoderm. Schaper's experiments indicate, I think, that the 

 time of the lens reaction is of considerable duration and if disturbed, 

 abnormal development of the lens results. 



We have noted in the conclusions that various portions of the ecto- 

 derm possess at a certain stage the lens-forming power. Are all por- 



