carpenter: development of the oculomotor nerve. 197 



the fundament of the posterior rectus muscle. Then the nerve, upon 

 reaching the region of the ciliary ganglion, which is ventral to it, sends 

 to this ganglion a strong fibrillar communicating branch (Plate 7, Figure 

 25, parasagittal section, rm. comn.), indications of which were present 

 in the preceding stage. The ophthalmic branch in its course next passes 

 immediately ventral to the median end of the dorsal rectus muscle ; 

 then, passing on the dorsal side of the distal end of the optic stalk, and 

 keeping close to the eyeball, it runs to the anterior region of the head, 

 where it breaks up into a number of slender branches. 



Ganglia are not to be seen along either the right or the left ophthalmic 

 branches, although ganglion cells occur here and there along their 

 courses. The ganglionic swellings observable in Stage IV have com- 

 pletely disappeared. At only two points on each nerve are there accu- 

 mulations of ganglion cells, and these are so small as to cause no 

 enlargement of the nerve trunk. The ganglionic groups occur at the 

 origin of the communicating branch (Fig. 2o, (3), and at the origin of 

 a small ramus which extends into the mesenchyme opposite the dorsal 

 rectus muscle. Both of these localities are far proximad of that of the 

 transitory ganglion of Stages III and IV, no trace of which now remains. 

 It will be remembered that in Stage IV one of the two ophthalmic 

 nerves showed a disorganized transitory ganglion, which, as was sug- 

 gested, might be considered in process of disintegration. 



In a series from a chick incubated one hundred nineteen and one-half 

 hours, the communicating branch between the ophthalmic division of 

 the trigeminus and the ciliary ganglion is of considerable size, its di- 

 ameter on the right side of the body, where it is a single trunk, being 

 rather more than one-fourth that of the ophthalmic branch. On the left 

 side, it consists of two separate bundles of til nils. In both cases it presents 

 the same appearance as the trunk of the ophthalmic branch, being made 

 up of fine fibrils with elongated "accompanying"' cells. The very small 

 number of migrant ganglion cells to be found along the communicating 

 branch is significant. Instead of affording a highway along which quan- 

 tities of cells from the ophthalmic branch pass over into the third nerve 

 to form the ciliary ganglion, it serves for the transit of very few of these 

 cells. On the left side of the body there can be counted along the com- 

 municating branch only five ganglion cells, while, on the right side, only 

 one undoubted cell of this nature can be made out. In the trunk of the 

 ophthalmic branch, at the place of origin of the communicating ramus, 

 there are present, on the left side of the body, twenty-one ganglion cells ; 

 on the right side, only two. 



