281 



LEKiH HOADI.I.V. 



thirty-six hour embryo will be considered next. At this stage the 

 optic primordium possesses great power of independent growth 

 and differentiation. Many grafts were obtained and sectioned, 

 a number of the most successful of which will be discussed here. 



FIG. i . Graft of an eye from a forty-eight hour chick embryo in alcohol before 

 section, 19 E 13. (X 10.) For explanation, see text, page 290. 



All of the cases which show any growth at all, prove, on micro- 

 scopical examination, to have a primary differentiation of the 

 tissues into a pigmented tapetal portion, a non-pigmented retinal 

 portion, and a group of lens-forming cells. The results vary all 

 the way from grafts in which there is no great differentiation of 

 the retina and lens, to the more successful ones in which the 

 retina is differentiated and the whole transplant appears approx- 

 imately normal. 



The more successful grafts show a high degree of differentiation 

 though great variation in the extent to which the subordinate 

 parts of the organ have differentiated. It is an interesting fact 

 that the basal layer of the retina often shows many cells in active 

 mitosis, i.e., the graft is still in active growth. In the best graft 

 obtained (Fig. 2), there is a very clearly defined retina with t he- 

 layers developed sufficiently to warrant the statement that it is as 

 far advanced as the normal; the lens is perfect, showing the 

 anterior epithelial layer and the lens nucleus; the anterior and 

 posterior chambers are present; the tapetum and tin- i artilage 



