carpenter: dkvelopment of the oculomotor nerve. 201 



nerve, have now drawn away from it. Fibrils can be detected turning 

 aside from the trunk of the oculomotor and entering tlie fundament of 

 the dorsal rectus. The nerve also sends a bundle of fibres to the funda- 

 ment of the ventral rectus, which has now become distinct from that of 

 the anterior rectus. The branch of the oculomotor which in the adult 

 innervates the latter muscle must develop later, since I have not been 

 able to detect it in three series of sections of about seven days' incuba- 

 tion. At this time, then, the third nerve exhibits all of the branches 

 found in the adult, with the single exception of that to the anterior 

 rectus muscle. 



The first indications of the formation of the sheaths of Schwann were 

 discovered only in the oldest embryo I have examined — one of eighteen 

 days' incubation. In longitudinal sections of the oculomotor stained by 

 the van Gieson method, it is apparent that the elliptical " accompanying " 

 cells lying among and parallel to the fibrils are sending out cytoplasmic 

 processes from both ends. These processes have not, however, developed 

 the longitudinal laminae, which mark the next step in the formation of 

 the sheaths of Schwann in mammals, as was first shown by Gnrwitsch 

 (:00). Bardeen ( :03) using the van Gieson stain, has confirmed the 

 conclusions of Gurwitsch. When present the laminae, exhibiting their 

 cut edges in cross-sections of the nerve fibrils, separate the latter into 

 bundles which are later bound closely together to form the neuraxons. 

 Cross-sections of the oculomotor nerve at this stage give no evidence of 

 the subdivision of the fibrils into such bundles. 



2. Ophthalmic Branch of the Trigeminal Nerve. The direct connection 

 between the ophthalmic branch and the ciliary ganglion persists at least 

 up to the eighteenth day of incubation, which is the oldest series in 

 which I have looked for it. It will be remembered that in the adult 

 such a direct connection does not exist, tlie communicating branch 

 entering the ciliary nerve some distance distal to the ciliary ganglion. 

 (Plate 1, Fig. 2). 



3. Ciliary Oanglion. The ciliary ganglion retains throughout life 

 its situation immediately on the trunk of the oculomotor nerve ; that 

 is to say, no radix brevis is ever developed. In fact it often appears, 

 even in the adult, as though the main trunk of the oculomotor ends in 

 the ciliary ganglion, while its continuation to the ventral oblique muscle 

 has the appearance of a large branch leaving the trunk just before the 

 ciliary ganglion is reached. It is, however, more in accordance with the 

 facts of comparative anatomy to regard the nerve as dividing into two 

 main branches at the region of the ramaus to the dorsal rectus muscle. 



