182 bulletin: museum of compakative zoology. 



the medullary wall are numerous, and actively engaged in the production 

 of indifferent cells. 



Along the course of the nerve the " accompanying " cells may fre- 

 quently be seen in process of mitosis, so that in case these are, as I be- 

 lieve, medullary and not mesodermal elements, it does not follow that 

 the whole number present at this time have migrated from the neural 

 tube, since their numbers are constantly increasing through cell division. 

 A vom Rath preparation of the root of the oculomotor, in an embryo 

 ninety-three hours old, show's a distinct difference in staining qualities 

 between the "accompanying" nuclei and the adjacent mesodermal 

 nuclei. While the "accompanying" nuclei take the stain readily, and 

 are sharply defined, the surrounding mesodermal nuclei are, owing to 

 their paleness, much less conspicuous (riate 6, Fig. 20). On the other 

 hand, the nuclei of the medullary wall show in their affinity for the 

 stain a striking resemblance to the "accompanying" nuclei. 



It will be noticed that both the nuclei within the neural tube and 

 those lying in the root of the oculomotor are more or less rounded in 

 form. At least few, if any, exhibit a pronounced elongation. However, 

 as we pass distally along the nerve, we find the most of the nuclei be- 

 coming more and more elongated, until the great majority are distinctly 

 spindle-shaped. A few cells, distributed along the entire length of the 

 nerve, retain the rounded form. 



Taking into consideration the whole course of the nerve, the greatest 

 amount of cell division occurs in the enlarged terminal (distal) portion 

 (Plate 5, Fig. 15), the fate of which proves it to be the fundament of 

 the ciliary ganglion. Here, the majority of the nuclei are not as elongated 

 as those lying among the fibres of the trunk of the nerve, many show- 

 ing approximately circular outlines. All are nearly destitute of cyto- 

 plasm at this stage. It is evidently owing to the proliferation of these 

 nuclei lying among 'the terminal nerve fibres that the enlargement in 

 this region has taken place. 



2. Ophthalmic Branch of the Trigeminal Nerve. The first branch of 

 the fifth nerve (Plate 7, Fig. 24, Plate 4, Fig. 12, rm. opth. trig.) passes, 

 in this stage, straight cephalad from the mesocephalic ganglion along 

 the lateral wall of the anterior cardinal vein (m. crd. a.). It termin- 

 ates dorsad of the optic stalk between the fore-brain and eyeball, and 

 just caudad of the laterally projecting vesicle of the hemisphere of that 

 side. Its distal extremity is marked by a transitory fusiform ganglion 

 (Plate 4, Fig. 12, gn. t'i.), having an approximate length of 165 roicra, 

 and a greatest diameter of 75 micra. This Figure is drawn from several 



