196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



That the number of the fibrils arising from the splitting up of neu- 

 raxous is increased distally along the nerve by longitudinal division of 

 the fibrils, can be asserted with a fair degree of confidence. It is pos-r 

 sible to see in the preparations strong indications of such branching, 

 though, as the fibrils concerned are very fine and lie closely associated 

 with one another, it is difficult in most cases, even with the highest 

 powers and most careful focusing, to be perfectly sure that the two 

 branches into which the fibril appears to divide are really the continua- 

 tions of that fibril. 



In preparations fixed in Zenker's fluid and stained with iron haema- 

 toxylin the finely fibrillar condition of the entire nerve is plainly demon- 

 strated, for not only are the identities of the neuraxons lost in the mass of 

 fibrils in the distal and middle parts of the nerve, but they are with diffi- 

 culty made out close to the root, where the fibrillar condition also appears. 



It is interesting to compare with the conditions in the chick longi- 

 tudinal sections through the oculomotor nerve of a pig embryo measur- 

 ing 8 mm. (its greatest length in its normal curved position). This 

 material was fixed in the corrosive-acetic mixture mentioned on p. 176 

 and stained with Brazilin. In Plate 4, Figure 14, A, the horizontal line 

 (n/b. lira. ear.) represents the external limiting membrane of the mid- 

 brain, and above it, on its way through the ventral fibre tract, is seen the 

 proximal end of an oculomotor neuraxon. Upon leaving the neural tube, 

 the neuraxon gradually grows thicker as it proceeds peripherally, and its 

 fibrillation becomes more marked. It can be followed as a distinct pro- 

 cess as far as has been indicated in the drawing, but at this point it comes 

 into relation with other neuraxons forming the root of the oculomotor, 

 and its identity becomes lost. The lower drawing, B, is that of a longi- 

 tudinal section through the same nerve about midway in its course. It 

 is plain that here the separate neuraxons cannot be distinguished. 

 The wdiole nerve is simply a homogeneous bundle of fibrils. 



A noticeable difference between the pig and the chick is the almost 

 complete absence of nuclei among the nerve fibrils of the former. A 

 few cells are to be seen lying along the periphery of the nerve, but it is 

 only occasionally that one can be found among the fibrils in the interior 

 of the bundle. 



2. Ophthalmic Branch of the Trigeminal Nerve. The Gasserian 

 ganglion is still bifurcated distally, but the two parts are more com- 

 pletely united proximally than in earlier stages. The ophthalmic branch 

 extends forward from the extremity of the tapering mesocephalic gan- 

 glion, passes just mediad of the anterior cardinal vein, and dorsad of 



