The Reactions of tlie Vertebrate Embryo to Stimulation etc. 575 



markable contrast that Ì8 present in the early stages between the 

 rapid and marked degree of ueurofibrillation in the meduUa, parti- 

 cularly about the nuclei of the Trigeminus, Acustico-facial, Glosso- 

 pharyngeus and Vagus, in comparison with the almost complete ab- 

 sence at this time of any evidence of this process within the brain. 

 At the same time the spongioblast elements are quite as advanced 

 in their development in the higher as they are in the lower centres. 



A review of the principal points brought out by the study of 

 the finer histological relations leads to the following conclusions: 



1) At the time when the neurofibrils begin to appear intimate 

 connections exist between many, if not all the cells, within the spinai 

 cord. Wbether this union is a genuine syncytium or the result of 

 fusion of material ejected from the cells cannot yet be definitely 

 decided. Even after the dififerentiation of neuroblasts and spongio- 

 blasts has begun, it is stili possible to detect connections existing 

 between these two dififerent types of cells. 



2) In Prìstiurus^ Scyllium and Torpedo embryos neurofibrillation 

 begins about the same time at the following places: In the large 

 cells of Beard dose to the nuclei, at the point of exit of the ventral 

 roots, within the bridges which connect the cord and the myotome 

 or within the myotome itself. 



3) The neuro-reticulum formed within the neuroblasts, as re- 

 presented by Held, is not the earliest stage in the development of 

 the fibrils as at first they are always parallel and bave no cross 

 connections. 



4) The primitive fibrils are coarse thick bundles which soon 

 split up longitudinally as originally described by Heidenhain and 

 Apathy. From the proximal end of one of these primitive bundles 

 fibrils may be seen to go to dififerent medullary cells at some 

 distance from the ventral-root group or are lost to sight in the outer 

 marginai veil of the cord. 



5) The connections formed by the neuro-reticulum between neuro— 

 Wasts are numerous and are already present in embryos of only 

 5mm. in length. 



6) The methods of staining generally employed fail to dififeren- 

 tiate between the neurofibrils and the protoplasm forming the celi 

 process. Although neuroblasts may throw out proeesses of con- 

 siderable length, as in the case of the Oculomotorius, in well diflferen- 

 tiated sections it may also frequently be seen that the protoplasm 



Mittheünngen a. d. Zool. Station zn Neapel. 6d. 18. 37 



