909 



bulletin: museum of comparative zoology. 



More theoretical and farther from the truth Eabl could scarcely be, yet 

 several investigators have in essential respects confirmed his theory, 

 that the oculomotorius is a derivative of the nem-al crest. Dohrn's 

 ('91) observations, however, differ fundamentally from those of his 

 predecessors. He sees the beginnings of the oculomotorius in em- 

 bryos intermediate between Balfour's stages I and K. At first cells in 

 the base of the brain assume a more transparent appearance, and later 

 migrate into the " Randschleier," where they send out processes which 

 unite in a network just outside the base of the brain to form the stem of 

 the nerve. Immediately at the beginning of the outflow of the plasma 



cells are seen half in and half out of the wall of the tube, and later, 

 but before the oculomotorius has any connection with the mesocephalic 

 ganglion, large deeply staining nuclei are seen in the protoplasmic net- 

 work which forms the root of the nerve. Dohrn does not lay great 

 stress on the fact that these nuclei are larger than those of the surround- 

 ing raesenchyma cells, but from the fact that similar nuclei lie nearer 

 the medullary wall, from which they appear to emerge in increasing 

 numbers during the course of development, he holds the opinion to be 

 permissible, that the nuclei of the early network are emerged raedullai-y 

 elements, and not mesoderm cells which press close to the medullary 



Fig. G. a portion of the same section as that shown in Figure F. X 240. 

 Tlie fibrillar nature of the oculomotorius is clearly shown, ax-cyl, axis cylinder 

 process ; cl. nil., neuroblast cell ; oc-mot., oculomotorius. 



