200 



Scientific Proceedings (108). 



region of the embryo. The present experiments upon Ambly stoma 

 show that the results may be different in the two cases. 



Excision of the eye rudiment in the medullary plate stage is 

 followed by suppression of the lens. Likewise, if the optic vesicle 

 is removed immediately or shortly after closure of the medullary 

 folds, the lens fails to develop, as shown by Le Cron. 



If, however, this same lens ectoderm is transplanted to other 

 regions of the head, a well differentiated lens will develop, pro- 

 vided the ectoderm is taken from the eye region after closure of 

 the medullary folds. Contact between optic vesicle and ectoderm 

 has at this time been established, though the two are not adherent 

 and may be readily separated without cells from one layer sticking 

 to the other. If the lens ectoderm is taken in earlier stages, small 

 and not fully differentiated lenses are sometimes but not always 

 formed. 



Barring one or two questionable cases, there is no evidence 

 that, in Ambly stoma, ectoderm from other parts of the head or 

 from the trunk can give rise to a normal lens. When such ecto- 

 derm is transplanted to the eye region, even before closure of the 

 medullary folds, abnormalities in the optic cup, due to irregular 

 infolding, frequently arise and no lens develops. When a circular 

 piece of ectoderm, having the diameter of the optic vesicle, is 

 removed from the eye region, the surrounding ectoderm, which 

 pushes in and covers the wound, usually gives rise to a lens, as 

 Spemann found to be the case in Triton. When larger pieces of 

 ectoderm are removed, the lens usually does not develop. 



These experiments show that Ambly stoma must be added to 

 those forms in which the lens ectoderm is capable of self-differen- 

 tiation. Why this power is manifested only when it is removed 

 from its normal position and not when it is left in place after 

 removal of the optic vesicle is problematical. The difference in 

 behavior can scarcely be referred to differences in the degree of 

 injury to the cells, but it is apparent that at times secondary cir- 

 cumstances of some unknown character may dominate more fun- 

 damental ones and thus lead to mistaken conclusions. 



