CARPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 195 



A study of the oculomotor with high powers at this stage gives evi- 

 dence of different histological conditions at different points along the 

 nerve. These conditions are well brought out by the vom Rath method. 

 A heavy black precipitate along the neuraxons differentiates these 

 clearly against the less darkly colored stroma in which they appear to 

 be imbedded. 



The drawings shown in Plate 2, Figure 7, A, B, C, are from a chick 

 of one hundred eighteen hours' incubation, stained by the vom Rath 

 method. These drawings were made from different parts of the nerve : 

 A, from the proximal end, B, farther distally, and C, near the peripheral 

 termination. All were made with the aid of a camera lucida under pre- 

 cisely the same conditions of magnification. At A, are shown two 

 neuraxons taken from the root as they appear under the -jV oil-immer- 

 sion lens. It will be seen that these are compact cylinders in which 

 fibrillation cannot he very satisfactorily made out, although indications 

 of it are to be detected here and there, for instance, in the upper portion 

 of the neuraxon lying on the right in the drawing. Closely applied to 

 the neuraxons are numerous elongated "accompanying" cells. 



Farther out on the nerve, in that portion which passes along the ven- 

 tral margin of the common fundament of the ventral and anterior rectus 

 muscles (comp. Plate 7, Fig. 26), the conditions are different. From 

 this part of the nerve I have drawn two bundles of fibrils with the ac- 

 companying embryonic nuclei of Schwann's sheath. These possibly rep- 

 resent two neuraxons, though it is impossible to trace any one neuraxon 

 with certainty through the series of sections from the root of the nerve 

 to this place. A careful study of the preparations has, however, con- 

 vinced me that the compact neuraxon which passes out from the brain 

 into the root of the nerve becomes more and more expanded and more 

 evidently fibrillar toward its peripheral extremity. In fact, close to 

 this extremity the fibrils become so numerous that those belonging to 

 the various component neuraxons of the nerve are inextricably inter- 

 mingled. The appearance of the oculomotor upon reaching the ventral 

 oblique muscle is shown at C, Figure 7. The bundles of fibrils which 

 are the continuations of the comparatively large cylindrical neuraxons 

 of the proximal portion of the nerve have here, at its distal end, lost 

 their identity in the mass of fibrillar elements. Nevertheless, while the 

 separate bundles of fibrils corresponding to the neuraxons are not well 

 defined, there can be seen in the nerve a tendency toward the formation 

 of more or less well-marked groups of fibrils, each of which may repre- 

 sent in the main a single neuraxon. 



