cakpenter: development of the oculomotok nerve. 183 



sagittal sections of a vera Rath series, and represents about one-half the 

 length of the ophthalmic branch of the trigeminus. In this particular 

 case, the nerve, just after passing the level of the optic stalk, divides 

 into two branches, which unite again at the ganglion ; but I do not find 

 this condition to be a constant one. 



An examination of longitudinal sections of the ophthalmic branch of 

 the trigeminus at this stage shows that, beside the elongated " accom- 

 panying" cells, resembling those of the oculomotor, there are to be 

 found distributed along the whole length of the nerve, as well as in the. 

 transitory ganglion, ganglionic cells whose larger nuclei and more abun- 

 dant and deeply staining cytoplasm make them easily distinguishable 

 from the "accompanying" cells. The description of these ganglionic 

 cells, and the discussion of the terminal ganglion, will be taken up under 

 Stage IV. 



In the transverse series of an eighty-eight-hours' chick there can be 

 seen at this stage, on the left side of the body, an exceedingly slender 

 offshoot of the ophthalmic branch of the trigeminus, passing to the 

 fundament of the ciliary ganglion. The offshoot is apparently composed 

 of a single neuraxon, to which a few "accompanying" cells are applied, 

 and at the place of its origin from the ophthalmic branch lies a cluster 

 of ganglion cells. On the right side of the body, on the contrary, the 

 most careful search has failed to reveal any fibrous connection what- 

 ever with the fundament of the ciliary ganglion. Of interest, however, 

 are two cells having the appearance of ganglion cells, both of which lie 

 in the mesenchyme between the ophthalmic branch and the fundament 

 of the ciliary ganglion, one being about midway between the two struc- 

 tures, and the other close to the surface of the ganglionic fundament. 

 Both these cells differ markedly from mesodermal cells, and also from 

 those of the fundament of the ciliary ganglion, on account of the larger size 

 of their nuclei and their deeply staining cytoplasm ; while their resem- 

 blance to ganglion cells seen in the ophthalmic branch, in the same sec- 

 tions, is very close. They appear to be migrating ganglion cells, and 

 their origin from the ophthalmic branch seems yery- probable. Further 

 evidence that ganglion cells do migrate from this nerve will be presented 

 in Stage IV. 



3. Abducent Nerve. The sixth nerve was first met with in a seventy- 

 eight-hours' series, that is, in a stage intermediate between StagSs II and 

 III. It was observed on both sides of the head as a very small nerve, 

 arising by five delicate roots from the ventral surface of the hind-brain, 

 about 135 micra from the median plane. The nerve can be traced 



