180 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The maxillary and mandibular branches of the trigeminus are poorly de- 

 veloped at this stage, 



3. Eye Mmclen. An isolated and compact accumulation of cells, 

 staining deeply with haematoxylin, lies in the mesenchyme along the inner 

 border of the anterior cardinal vein, not far from the ventro-lateral face 

 of the anterior portion of the hind-brain (Fig. 23, mu. rt. p.). The dis- 

 tal extremity of the second division of the Gasserian ganglion lies lat- 

 erad to this cell group, separated from it by the lumen of the anterior 

 cardinal vein. The subsequent development of the mass proves it to be 

 the fundament of the posterior rectus eye-muscle. No other eye-muscle 

 fundaments are present, and not even the nidulus of the abducent nerve, 

 which later innervates the posterior rectus muscle, has as yet appeared. 



Stage III. 



This stage is represented by two series of preparations, one of eighty- 

 eight hours', the other of ninety-three hours', incubation. The two em- 

 bryos had attained practically the same degree of development. 



1. Oculomotor Nerve and Ciliary Ganglion. A fortunate section from 

 the eighty-eight-hours' series, taken tran.sveraely to the axis of the mid- 

 brain, shows the oculomotor cut longitudinally throughout its whole 

 length (Plate 7, Fig. 24, n. oc^mot.). On either side of the median 

 plane, and lying in the ventral wall of the mid-brain, the oculomotor 

 uiduli {nidi, oc'mot.) are seen as elliptical groups of cells, delimited 

 by narrow areas comparatively free from medullary elements. These 

 niduli present an elliptical outline whether viewed in transverse or 

 parasagittal sections, but the ellipse seen in the latter plane has the longer 

 principal diameter. Each nidulus measures approximately as follows : 

 longitudinal diameter, 450 mi era ; transverse diameter, 135 mi era ; ver- 

 tical diameter, 78 micra, the latter being more than half the thickness of 

 the mid-brain wall, which here measures 117 micra. Tlie centres of these 

 niduli are slightly over 150 micra from the median plane. Allowing for 

 increase in size due to growth, it is apparent that little, if any, change 

 has taken place in their positions since their first appearance in Stage I. 

 They then lay at the distance of 100 micra from the median plane. 



The marginal veil is now invaded by the developing ventral fibre-tract, 

 which forms a comparatively bi'oad band of separation between the ordi- 

 nary cells of the medullary wall and the external limiting membrane. 

 The cells of the nidulus, however, project into this region, and extend to 

 the very margin of the wall. Not all the neuraxons given off by the 

 neuroblasts of the oculomotor nidulus pass out into the root of the 



