186 bulletin: museum of comparative zoology. 



straight course, passing ventrad and slightly laterad (Plate 4, Fig. 13) 

 through the mesenchyme, between the infundibulum on its median, and 

 the lower half of the eyeball on its lateral side. The large accumulation 

 of cells forming the ciliary ganglion lies mainly on the lateral, or ocular, 

 side of the nerve trunk (Compare Plate 7, Fig. 25 gn. ciL). The neu- 

 raxons of the nerve continue for a short distance beyond the ganglion, 

 and, bending somewhat laterad, terminate immediately mediad of the 

 antero-ventral portion of the eyeball in the fundament of the ventral 

 oblique muscle, which makes its first appearance in this stage. The 

 oculomotor is as yet entirely without branches. 



A histological change has by this time taken place in, at least, the dis- 

 tal two-thirds of the nerve. In early stages, especially in vom Path 

 preparations, the neuraxons of the nerve are to be seen, under high 

 powers of the microscope, as relatively thick fibres with peripherally 

 situated " accompanying " cells. The same powers now show, especially 

 in the more distal parts of the nerve, that the relatively thick fibres no 

 longer appear, their places having been taken by much finer fibrils. As 

 a consecpience, the fibrous components of the nerve are now greatly in- 

 creased in number without a corresponding increase in the calibre of 

 the nerve. I shall hereafter speak of these fine filaments as fibrils in 

 contradistinction to the earliei\/77;m? — the coarser structures which the 

 fibrils have replaced. The histogenesis of the fibrils is considered under 

 Stage V. 



Lying at all depths within the nerve, " accompanying " cells may be 

 seen closely applied to the fibrils. In the description of the preceding 

 stage, evidence was brought forward to show that these "accompanying" 

 cells have been derived through migration from the neural tube, where, 

 it is maintained, they originate as rounded, indifferent cells, the descend- 

 ants of the germinative cells, and where, according to Schaper, they may 

 later differentiate into either nervous or supporting elements, i. e., neu- 

 roblasts or spongioblasts. Certain of these cells, through their power of 

 locomotion, are capable of leaving the central nervous system, and, fol- 

 lowing the path of the neuraxons, of reaching the peripheral nerve trunk. 

 Lying among the fibrils of the nerve, they increase in number by divi- 

 sion. Knowing, as we do, the subsequent history of the indifferent cells 

 remaining within the neural tube, an analogous fate might be expected, 

 a priori, in the case of those which migrate out into the nerve trunk. 

 That many of these emigrant indifferent cells do eventually subserve a 

 supporting function, not in the form of neuroglia, but as the sheaths of 

 Schwann, can hardly be doubted. Such cells become elongated soon 



