Experimental Studies ofi the Development of the Eye. 435 



If at a much later stage, namely, shortly after separation of the 

 lens from the ectoderm, both optic cup and lens are taken out in a 

 manner similar to that just described, corneal changes fail to 

 develop, even if the embryos are allowed to live from twelve to 

 eighteen days after the operation. (See Fig. i, from Experiment 

 XIV3,,.) 



The end result as regards non-development ot the cornea is 

 the same in each embryo, whether the eye is taken out before the 

 lens begins to form or shortly after its separation from tbe skin. 



In Amblystoma, as in Rana, however, corneal formation occurs 

 after partial extirpation of the eye, whether this is done before the 

 lens has formed or shortly after its separation from the ecto- 

 derm, provided the regenerated eye comes into contact with the 

 ectoderm. The mere lifting of the skin-flap here as in Rana does 

 not interfere with corneal formation, so that the lack of corneal 

 development after complete extirpation of the eye is not due to 

 the lifting of the skin-flap but must be in some manner associated 

 with the absence of the eye. It is evident that the cornea is not 

 a self-differentiating structure. 



B. Rudimentary Corneal Area after Late Extirpation of the Eye. 



If in Amblystoma the optic cup and lens are taken out some- 

 time after the separation of the lens from the ectoderm but before 

 corneal changes are visible on the surface of the embryo a small 

 clear corneal area will develop in the region w^here the large 

 normal cornea would have formed. An examination of a normal 

 embryo of the same stage at which the operation was performed 

 shows that corneal changes have begun and consist in a slight 

 thinning of the ectoderm over the eye. The endothelial layer 

 is also in the process of formation. The operations consist in 

 making an incision about the caudal side of the eye and then 

 carefully turning forward the skin-flap from over it without injury 

 to the corneal region. The eye and lens were then cut out and 

 the skin-flap turned back into place where it readily healed. 

 A few days after the operation a small clear corneal area appears 

 in the ectoderm of the corneal region. An examination of the 

 sections shows that this clear area differs from the ordinary 

 ectoderm surrounding it in that it is much thinner, the pigment 

 is wanting and the ectodermal cells have lost or not acquired the 



