JO8 LEIGH HOADLEY. 



cells of the host, between which the nerve processes make their 

 way. At numerous points along the periphery these processes 

 are united into nerve-like masses which end bluntly at varying 

 distances from their point of origin. Some of the cell bodies of 

 the outer nuclear zone appear to have migrated through the 

 immediately surrounding loose mesenchyme to a more definitely 

 organized envelope of mesenchymal tissue which surrounds the 

 graft and in which run the blood vessels of the host. The struc- 

 ture of this sheath resembles very closely that of the correspond- 

 ing layer in the normal embryo. It is impossible to say whether 

 it arose from the mesenchyme of the host or of the graft. The 

 blood vessels are those of the host and they show a most extensive 

 development. In places, large spaces filled with blood are 

 present between this layer and the developing mesencephalon. 

 Apparently because of the fact that the external limiting mem- 

 brane is not present in such places, the vascular infiltration of the 

 fiber layers as well as the nuclear layers is enormous. Even in 

 the mesocoele there are large groups of blood cells though no 

 organized vascular walls appear around them. In the nuclear 

 layers there is a crowding of the developing nerve cells due to the 

 presence of small vessels. In spite of this fact, the layers are 

 distinct. The individual cells of the areas seem to be differenti- 

 ating in a normal manner. 



The significance of the absence of the external limiting mem- 

 brane becomes evident upon examination of another graft, 18 B 

 5 (Fig. 1 1), in which this membrane is present around a majority 

 of the cortical tissue. Where it is present, the differentiation of 

 the layers of the cortex corresponds exactly with that of the 

 control. In the peripheral fiber zone, the fibers form a clear band 

 and do not extend into the mesenchyme. As a result, none of the 

 nerve-like processes are found. The vascularization of such 

 regions is much less extensive than that of regions where the 

 membrane is absent. As a result of this, practically no distortion 

 of the nuclear and fiber layers occurs. These are normally pro- 

 portioned and contain cells which are in the process of normal 

 differentiation. 



In other portions of the same graft there are areas lacking the 

 external limiting membrane. As w r e should expect, the outer 



