CARPENTER :. DEVELOPMENT OF THE OCULOMOTOR NERVE. 179 



motor uiduli, lying in the ventral mid-brain wall, with their centres about 

 120 micra from the median plain. The niduli invade the marginal veil, 

 which is now well developed. From the neuroblasts, cytoplasmic pro- 

 cesses run out into the mesenchyme, as may be seen in Plate 3, Figure 

 9. These appear to blend more or less, and, outside the neural tube, 

 make a network with many nuclei lying along the threads of the net. 

 This reticulated appearance of the nerve during its early development 

 always follows fixation in Zenker's fluid or the corrosive-acetic mixture. 

 In the present instance, the material was fixed in Zenker's fluid, and 

 stained with Brazilin. Vom Rath preparations, on the contrary, exhibit 

 the neuraxons of the growing nerve as separate elements, approximately 

 parallel to one another, and not connected to form a network (comp. 

 Plate 6, Fig. 20). 



The cells lying along the nerve strands are composed of rounded nuclei 

 with which very little cytoplasm appears to be associated. For the sake 

 of convenience, these cells will hereafter be called accompanying cells. 

 Their nuclei resemble closely in form, size and staining qualities, the 

 nuclei of the indifferent cells which lie inside the neural tube. Certain 

 of the indifferent cells of the nidulus may be seen in the process of di- 

 vision (Plate 3, Fig. 9, cl.'), and many lie near, and indeed in some cases 

 (cl.") immediately in contact with, the external limiting membrane. 



The oculomotor may now be traced for some distance by its cytoplasmic 

 threads and " accompanying " cells. It pursues a straight course through 

 the mesenchyme in a ventral direction from the mid-brain. On account 

 of the cephalic flexure, it lies nearly parallel to the axis of the hind-brain. 

 After proceeding a short distance, it terminates in a slightly expanded 

 distal extremity. 



2. Ophthalmic Branch of the Trigeminal Nerve. In this stage the 

 Gasserian ganglion, which has of course been present since its differen- 

 tiation, much earlier, from the neural crest, shows plainly a partial divi- 

 sion into a mesocephalic ganglion, giving rise to the ophthalmic branch 

 of the trigeminus, and the ganglion of the maxillary and mandibular 

 branches. The mesocephalic division is directed cephalad, and from its 

 distal extremity the ophthalmic branch proceeds for a short distance along 

 the outer wall of the anterior cardinal vein in the direction of the eye- 

 ball, but 'cannot be traced as far as the level of that organ. The maxillo- 

 mandibular division of the Gasserian ganglion is directed ventrad and 

 laterad, making approximately a right angle with the mesocephalic 

 portion. It extends as far as the ectoderm, with a thickened patch of 

 which (Plate 7, Fig. 23, gn. mx-md. Gas.) it is directly connected. 



