CAEPENTEE : DEVELOPMENT OF THE OCULOMOTOE NERVE. 187 



after their escape from the medullary wall, and, during succeeding stages 

 of development, can be observed adhering closely to the nerve fibrils, 

 until finally, just before the hatching of the animal, they give evidence 

 of participation in the formation of the sheaths of Schwann. The other 

 possibility indicated by their identity with the indifferent cells of the 

 neural tube — namely, their differentiation, in part, into ganglion cells 

 — will be considered under the subject of the ciliary ganglion. 



In the one-hundred-hours' series there occurs on the oculomotor 

 nerve midway between its root and the ciliary ganglion, a group of cells 

 worthy of attention. As is shown in Plate 5, Figure 17 — which is 

 a longitudinal section through a portion of the oculomotor nerve — 

 these cells are placed at the margin of the nerve trunk, and present 

 a striking contrast to the "accompanying" cells lying along the fibrils. 

 While the nuclei of the latter cells are drawn out into oval, elliptical 

 and spindle forms, and possess very little cytoplasm, those of the cells 

 forming the group have mostly an almost circular outline, and lie 

 embedded in an abundance of granular cytoplasmic material, which 

 stains deeply with haematoxylin. Proximal and distal to this group 

 the nerve exhibits only fibrils and the ordinary elongated " accom- 

 panying" cells. I shall have occasion to refer again to this accumu- 

 lation of differentiated cells when discussing the ciliary ganglion. The 

 single layer of strikingly elongated cells at the periphery of the nerve 

 is probably made up of mesodermal elements. The processes of these 

 cells unite to form a thin envelope, which doubtless represents the 

 connective-tissue sheath or perineurium of the adult nerve trunk. 



2. Ophthalmic Branch of the Trigeminal Nerve. The transitory 

 ganglion which appeared in Stage III still persists, but in one series it 

 presents on the left side of the body a disorganized appearance, being 

 represented by small clumps of ganglion cells, which lie scattered about 

 in the mesenchyme in the immediate vicinity of the place on the 

 ophthalmic branch where one would expect to find the ganglion. 

 Possibly we have here the beginning of a process of disintegration. 

 The ganglion on the right nerve in the same series is as compact and 

 definitely limited a body as in the preceding stage, and it lies in the 

 same position, immediately posterior to the laterally projecting vesicles 

 of the cerebral hemispheres. The nerve continues beyond the ganglion 

 as a slender strand, which runs ventrad of the lateral vesicle of the 

 fore-brain, trending also laterad as it proceeds, and, before reaching the 

 level of the anterior extremitj' of the fore-brain, terminates in a small 

 swelling containing ganglion cells. Several small and rather indefinite 



