284 LEIGH HOADLEY. 



III. EXPERIMENTS. 

 A. The Eye. 



A series of grafts of the optic primordium of the chick have 

 been made by this method. The material was taken from chick 

 embryos of twenty-four, thirty-six, forty-eight, and in a few cases 

 sixty hours of incubation. In some cases the entire primordium 

 was excised, in others only a part of it was used. There are 

 various difficulties to be encountered in such an operation. The 

 mesenchyme, at these early stages, is quite sticky, and consider- 

 able care must be exercised to remove the greater part of it from 

 the vesicle. Particular care is needed if it is desired that no 

 part of the brain be included in the transplants. At the twenty- 

 four hour stage, the neural folds are open in the optic region and 

 only a slight swelling indicates the position of the optic foveola. 

 The anterior end of the head was removed from such embryos; 

 the region from which the optic vesicles develop was then bisected, 

 and the primordium of each side was transplanted separately. 

 The lens ectoderm is not yet definable so that transplantation of 

 this region with that of the future optic vesicle is a matter of 

 chance. At the thirty-six hour stage, the technic is much more 

 simple. Here the embryo has from twelve to sixteen somites. 

 Some of the embryos of this age were more advanced, but the 

 majority fall within these limits. The primordium of the eye is 

 then in the vesicle stage and is easily removed. Great care must 

 be taken that the tissue is not harmed in the transplantation. 



Another set of experiments was made on the eye at the thirty- 

 six hour stage. The main part of the experiment was the same, 

 but the lens ectoderm was dissected away. This is very difficult 

 to do, for there is a great tendency for the wall of the optic vesicle 

 to tear. In most of the cases which have been operated in this 

 way, no lens ectoderm was left on the vesicle, but one case which 

 later developed small portions of lens tissue is in doubt. The 

 mechanical injury in an operation, must, of necessity, be great, 

 and this is probably the cause of the greater degree of disorganiza- 

 tion found in these grafts. Grafts of the isolated lens ectoderm 

 were also made but no growth was found in any case, possibly 

 owing to the very small size of the piece of the embryo trans- 

 planted. 



