cabpentee: development of the oculomotor nerve. 199 



longitudinally elongated nidulua in the floor of the hind-brain by live or 



more roots placed one behind the other (Plate 6, Fig. 18, u. abd.). These 

 roots often become divided near their region of emergence from the 

 neural tube, so that a strict count of their number is made difficult. 

 The number of roots differs in different individuals, and even on oppo- 

 site sides of the same individual. Although Figure 18 shows but live 

 roots, on the other side of the embryo six can be counted. This figure 

 shows the manner in which the roots unite to form the nerve, which is 

 now of comparatively large size, although previously it has been remark- 

 able for its small calibre. The nerve trunk (Plate 7, Fig. 26, n. abd.) 

 runs cephalad, laterad, and slightly ventrad to end in the fundament 

 of the posterior rectus muscle (uta. rt. j>.). In oue case, the nerve bi- 

 furcates just before reaching the fundament, and while one division 

 continues the course of the nerve straight into the muscle mass, the 

 other runs on the dorsal side of it for some distance, and then turn- 

 ing ventrad enters the fundament, dividing again as it does so. The 

 fibrils of the nerve can be traced far into the mass of differentiated 

 mesodermal cells constituting the muscle fundament. 



The evidence for cell migration into the roots of the abducens is still 

 good at this stage, the ventral fibre tract in the region of the nidulus 

 being less thick than where the oculomotor emerges. Figure 19 

 (Plate 6), drawn from a preparation fixed in the picro-sulphuric mix- 

 ture and stained with Delafield's hematoxylin, shows that rounded 

 nuclei in the wall of the neural tube extend down from the nidulus 

 into the ventral fibre tract, and, indeed, to the very surface of the 

 tube, where they become continuous with similar nuclei lying outside 

 the tube along the nerve. Farther out on the nerve these nuclei 

 become elongated. As has been said, all the indifferent cells escaping 

 in connection with the sixth nerve appear to assume a supporting func- 

 tion, developing into the sheaths of Schwann. At no time during the 

 growth of the nerve can young ganglion cells be discovered at any 

 place along its course. 



5. Eye Muscles. The section represented in Plate 7, Figure 2G, shows 

 in one plane the fundaments of the posterior rectus eye muscle and the 

 four eye muscles innervated by the oculomotor nerve. All the eve- 

 muscle fundaments consist of clusters of crowded elongated mesodermal 

 nuclei, the interstices between which are filled with differentiating cyto- 

 plasm, which stains deeply with luematoxylin. 



The first, in order of development, is the posterior rectus muscle 

 (jnu. rt. 2'-). This was present in Stage I, and its position and appear- 



