532 Experimental Studies on the Origin of the Lens in Amphibia 



places on the skin as a result of the contact of the optic vesicle of the 

 same animal, but the optic vesicle of one species may cause a lens to 

 arise from the ectoderm of another species of frog. 



3. Various portions of the optic vesicle can stimulate lens formation. 



4. We must conclude then that in normal development the lens is 

 dependent for its origin on the contact influence or stimulus of the optic 

 vesicle on the ectoderm. 



DISCUSSION. 



Are there an}^ conditions under which the lens can arise from the 

 ectoderm without the stimulus of the optic vesicle? 



If we include under the term optic vesicle the cells that under normal 

 conditions give rise to a distinct optic vesicle but under abnormal con- 

 ditions may never evaginate from the brain wall, we can at least say that 

 there is no proof to the contrary. Mend's " case, as Spemann suggests, 

 can best be explained by considering that the optic vesicle cells still 

 form part of the brain wall and that this part of the brain was at one 

 time in contact with the ectoderm, and thus stimulated the lens forma- 

 tion. Eabl's ' case can easily be explained by a shifting of the ectoderm 

 and newly forming lens away from the small optic vesicle. Schaper's * 

 experiments give somewhat similar pictures to Eabl's. The rudimentary 

 lenses in Schaper's experiments were undoubtedly caused by the shifting 

 of the ectoderm and lens, thus removing the newly forming lens from 

 the influence of the optic vesicle. In reality the examples of Mencl 

 and Eabl prove neither one side nor the other and the experimental evi- 

 dence is all directly for the idea that a lens will not arise from the skin 

 without the stimulus of the optic vesicle cells. Especially does this 

 seem evident if we consider that there is a definite reaction of some 

 kind between the optic vesicle and the ectoderm cells which give rise to 

 the lens. 



The fact that the optic vesicle becomes quite firmly adherent to the 

 ectoderm just preceding and during the early stages of lens formation 

 and until after its complete separation from the ectoderm, is very sug- 

 gestive, indicating perhaps that there is a protoplasmic connection of 



' Ein Fall von beiderseitigen Augenlinsenausbildung wahrend der Abwesen- 

 heit von Augenblasen. Archlv f. Entwickelungsmech., XVI. 



' Ueber den Bau und die Entwickelung der Linse, I. Zeit. f. Wiss. Zool., 

 Bd. 63, 1898, p. 530. 



* Ueber einige Falle atypischer Linsenentwickelung unter abnormen Beding- 

 ungen. Anat. Anz., Bd. XXIV, No. 12, Jan., 1904. 



