192 bulletin: museum of comparative zoology. 



or supporting elements. Many of these nuclei, once out on the nerve, 

 become elongated as they move away from the neural tube. Such " ac- 

 companying " cells maintain throughout development their close prox- 

 imity to the" nerve fibrils, and in them we recognize, as has been pointed 

 out, the nuclei of the future sheaths of Schwann. A large part, then, 

 of the emigrant cells become supporting elements. 



Do any of the emigrant indiiferent cells become nervous elements? 

 We have seen that at all stages rounded nuclei, resembling and continu- 

 ous with the indifferent cells of the neural tube, occur abundantly at the 

 root, and more sparingly along the trunk, of the oculomotor nerve, lying 

 among the more numerous elongated supporting cells. In Stage III an 

 accumulation of such cells was observed at the distal end of the nerve, 

 causing at this place its enlargement into the fundament of the ciliary 

 ganglion (Plate 7, Fig. 24, gn. eil.), the cells of which were undergoing 

 active division (Plate 5, Fig. 15). Schaper, it will be remembered, 

 shows that the indifferent cells of the central nervous system likewise 

 possess the property of further propagation. In the present stage, IV, 

 the ciliary ganglion of the right side contains undoubted ganglion cells. 

 The right oculomotor has not yet come into connection with any other 

 nerve, although the ophthalmic branch of the fiftii is sending fibrils in 

 the direction of the ciliary ganglion, and toward the latter a few oph- 

 thalmic ganglion cells are apparently making their way through the 

 mesenchyme. Three, in fact, lie just within the borders of the ciliary 

 ganglion, easily distinguishable by their larger size from the numberless 

 ganglion cells about them. The vast majority of the cells of the ciliary 

 ganglion, however, could have originated only by differentiation from 

 the rounded, proliferating cells which are to be seen in Stage III occu- 

 pying the site of the future ganglion, before there is the slightest trace 

 either of a connection between the oculomotor and the ophthalmic 

 branch of the trigeminus, or of the migration of ophthalmic ganglion 

 cells through the mesenchyme towai-d the fundament of the ciliary 

 ganglion. There is good evidence that the actively dividing cells of 

 the latter ganglion had their origin in indifferent medullary cells which 

 had escaped from the neural tube. If this be so, then a small portion 

 of the indifferent cells migrating out from the mid-brain do become 

 differentiated, after increase in numbers by division, into nervous ele- 

 ments, i. e., ganglion cells of the ciliary ganglion. 



An accumulation of differentiating cells at the side of the third nerve, 

 mi<l\vay between its root and the ciliary ganglion (Plato S, Fig. 17), 

 during this stage, has been referred to in the account of the oculomotor 



