442 Warren Harmon Leivis. 



G. Degeneration of the Cornea after Extirpatiori of the Optic Cup 



and Lens. 



If after the cornea is well formed an incision is made dorsal 

 to the eye and the optic cup with the lens taken out, care being 

 taken not to injure the cornea, the large cornea will gradually 

 disappear. At first instead of forming a bulge on the surface of 

 the head there results a depressed area owing to the absence of 

 the optic cup. At the bottom of this area is the corneal clearing. 

 This corneal area gradually contracts and pigment cells invade 

 it from the adjoining skin, and in some of the experiments there 

 is scarcely a trace of the cornea left 30 days after the operation. 



CONCLUSION. 



It seems very probable that the optic vesicle brings about lens 

 formation through a specific influence. The cornea, however, 

 in so far as its early stages are concerned, namely, the thinning 

 and clearing of the skin and loss of pigment can hardly be 

 ascribed to a specific influence. That the mechanical pressure 

 of the eye, or cup or lens may in some way be accountable for the 

 corneal changes is a possibility. I am more inclined to the 

 view, however, that the changes are due to another and quite 

 difi^erent reason. The contact of either the eye or cup or lens 

 with the epidermal cells must of necessity alter the environment 

 of the overlying cells as regards their relation to the mesenchyme. 

 It is possible that this exclusion from contact with the mesenchyme 

 cells may be responsible for changes in the metabolism of the 

 epidermal cells and cause them thereby to alter their mode of 

 development, such alteration leading to corneal changes. That 

 such apparently slight alterations in environment are responsible 

 for other important changes in the history of embryonic ecto- 

 dermal cells I think quite probable. From some experiments 

 already completed it seems highly probable that the central 

 nervous system is in part at least dependent for its origin and 

 differentiation on the diff^erence of environment of cells which at 

 one time possessed the possibilities of producing either ordinary 

 epidermal cells or of producing nerve cells. 



