194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



twenty hours. The cephalic flexure has heeome less pronounced, so 

 that the axis of the fore-brain makes with that of the hind-brain ap- 

 proximately a right angle, at the vertex of which the large vesicle of 

 the mid-brain projects externally. 



1. Oculomotor Nerve. The oculomotor nidulus is now about 200 

 micra from the median plane. The fibre tract of the ventral wall of the 

 mid-brain is well developed, forming a third of the entire thickness of 

 the wall. The processes of the oculomotor neuroblasts extend ventrad 

 through this tract, the fibres of which run at right angles to the pro- 

 cesses; Along the oculomotor neuraxons lie "accompanying" cells, 

 which resemble in every respect the cells which are present in great 

 abundance in the root of the nerve immediately outside the external 

 limiting membrane. 



However, within the neural tube the "accompanying" cells do not 

 often extend along the neuraxons all the way to the limiting membrane. 

 It appears that migration is by this time nearly over. The neuraxons of 

 the third nerve are closely interwoven with numerous nerve fibres run- 

 ning at right angles to them. The " accompanying " cells, which measure 

 approximately 4 micra in diameter, in order to pass from the mantle 

 layer to the mesenchyme, would be obliged to force their way along the 

 oculomotor neuraxons through a feltwork about 75 micra in thickness. 



From its root, which is still spread out longitudinallj', the oculomotor 

 nerve runs caudad, ventrad and somewhat laterad. On reaching the 

 level of the optic stalk, it turns toward the median -plane, and, continu- 

 ing its course ventrad and mediad for a short distance, ends in the ven- 

 tral oblique eye muscle. About two-thirds of the way from the 

 proximal to the distal end of the nerve, the unsymmetrically elliptical 

 ciliary ganglion is to be seen lying on the nerve trunk. This portion of 

 the nerve is seen in the parasagittal section shown in Plate 7, Figure 26 

 («. oc'mot.), which is viewed from the right face. The fundament of 

 the dorsal rectus eye muscle {mu. rt. d.) and the common fundament 

 of the ventral and anterior rectus muscles (rnu. rt. v. + a.) lie in close 

 contact with the dorsal side of the nerve, but no fibrils can be detected 

 turning aside from the main trunk to penetrate these muscle masses. 

 That is to say, so far as can be seen, the oculomotor remains at this 

 stage an unbranched nerve. 



Along the posterior margin of the root occurs a bundle of fibres ren- 

 dered conspicuous by their freedom from " accompanying " cells. Along 

 the external margin of this bundle a single layer of cells separates it 

 from the mesenchyme. 



