304 LEIGH HOADLEY. 



is identified as that of the otic capsule but it is very definitely the 

 result of the differentiation of the cartilage-forming tissue of tin- 

 graft and not of the host mesenchyme stimulated by the mem- 

 branous labyrinth. The very fact that it only partially sur- 

 rounds the parts of the vesicle proves this. It will be recalled 

 that a fringe of mesenchyme was transplanted together with the 

 otic vesicle. This would easily account for any cartilage present 

 in the grafts. 



The specificity of the cellular elements of the otic vesicle is very 

 marked (see also Streeter, he. cit.). Utriculus, sacculus, endo- 

 lymphatic duct and sac, and the canals are recognizable by the 

 character of their histological differentiation. Though the canals 

 have not formed in any of the cases, their general position is 

 indicated by the occurrence of the cristae acusticae of their ampullae. 

 In no case are they accompanied by ganglion cells. Neither is 

 the auditory nerve present in any of the grafts. This indicates a 

 complete isolation of the primordium, for the eighth ganglion is 

 closely associated with the otocyst even at the stage used in the 

 operations. There is therefore no possibility of a stimulation of 

 the epithelium of these sensory areas by nerve cells. These areas 

 are localised in the epithelium itself. By means of various graft- 

 ing experiments, Harrison (1903) tound a similar differentiation 

 of a sensory area, independent of nervous stimulation, to exist 

 in the development of the lateral line organs of Rana sylvatica and 

 Ran a palustris. 



In spite of the much altered relationships of the parts of the 

 membranous labyrinth and the surrounding mesenchyme as they 

 develop in the grafts, they differentiate so that they are easily 

 identified in the sections. The amount of adjustment of the 

 relationships of their parts is, on the other hand, very variable, 

 being dependent on the amount of mechanical injury and the 

 position of the grafted tissue at operation, together with the 

 factors of pressure, and stress and strain during the following 

 growth period. 



D. The MesencepJiahn. 



As has been stated above, the selection of the mesencepliulon 

 in making grafts of brain tissue was made because of the- fart th.it 

 it is here that the correlation centers of the eve have tlu-ir seat. 



