312 LEIGH HOADLKV. 



the present study indicate that it is at a very early stage. It is 

 conceivable that, for the eye, for example, specialization iiuy 

 occur after the earlier stages used by Danchakoff, for in her 

 experiments, the entire embryo with a part of the blastoderm and 

 not the isolated primordium was transplanted, so that physiologi- 

 cal isolation was not complete. The time of such organization 

 differs for different animal classes as has been shown by the 

 experiments on the independent development of isolated blasto- 

 meres, some cleavage being determinate from the very first with 

 no post-regeneration. The antero-posterior course of embryonic 

 differentiation in the chick indicates that the time of this speciali- 

 zation varies for different parts of the same embryo. 



The results of the study of the organs described indicate that 

 the cells of their parts possess a specificity which is histologically 

 equal to that of the normal. That such a specificity is an innate 

 property of the cells of the embryo at certain stages has, moreover, 

 been shown by numerous investigators by the methods of tissue 

 culture, where the cells behave more or less as independent units. 

 The same principles apply to the development of the subordinate 

 parts of the organs grafted. This is emphasized by the dif- 

 ferentiation of the cristae acusticae of the membranous labyrinth, 

 the lens and the lenticular zone of the eye, and the various parts 

 of the nasal region and the mesencephalon. The development 

 of a subordinate part when isolated as such, is a problem, the 

 results of which must be established by further investigation. 

 That such parts are capable of differentiation without the stim- 

 ulation of external factors is proven by their behavior when 

 isolated in the primordium of the organ. Xo nerve stimulation 

 of the cristae acusticae is necessary for their origin, for example, a 

 condition closely resembling that found to exist in the formation 

 of the lateral line sense organs of the amphibian larva by Har- 

 rison. The results which have already been obtained indicate 

 that the same general principles are involved in the differen- 

 tiation of the parts of the other primordia which have been 

 employed in the transplantations. 



The limits of development of the isolated parts seem to depend 

 on the mechanical factors involved rather than on the physiologi- 

 cal isolation as such. In none of the cases is this more marked 



