266 THE CHICK. 



early on the fourth day, its main branches of distribution become 

 definitely established. 



These branches, in the case of the seventh or facial nerve, 

 are closely connected with the hyo-mandibular cleft. They 

 consist of a large hyoidean or post-branchial branch (Fig. 115, 

 vn), which runs along the hyoicl arch ; and a smaller mandi- 

 bular or prebranchial branch, which runs forwards over the 

 dorsal end of the hyo-mandibular cleft, and then downwards a 

 short distance along the mandibular arch. 



The above account will apply to any one of what may be 

 termed the typical cranial nerves. It will now be convenient to 

 take the several cranial nerves one by one, and note the chief 

 points in their individual development. 



I. The olfactory, or first cranial nerve. Our knowledge of 

 the development of the olfactory nerve in the chick is still 

 incomplete in some respects. At the twenty-ninth hour the 

 neural ridges extend forwards along the brain as far as the 

 anterior end of the fore-brain, i.e. in front of the optic vesicles 

 (cf. Fig. 111). There are reasons for thinking that it is from 

 the anterior ends of the neural ridges that the olfactory nerves 

 are, at any rate in part, developed ; but the point has not been 

 proved by actual observation. 



At the fiftieth hour, before the paired cerebral hemispheres 

 have commenced to appear, the olfactory nerves may be recog- 

 nised as a pair of short outgrowths, arising from the dorsal 

 surface of the unpaired vesicle of the hemispheres, and running 

 downwards and outwards towards a pair of slightly thickened 

 patches of epiblast, on the under surface of the head, which form 

 the earliest rudiments of the olfactory pits. 



During the third day the cerebral hemispheres arise. These 

 are, from the first, situated dorsally to the roots of the olfactory 

 nerves; and, growing rapidly forwards and upwards (Fig. 113), 

 they appear to drive the olfactory nerves down to the base or 

 ventral surface of the brain. By the further growth of the 

 cerebral hemispheres the original unpaired vesicle of the hemi- 

 spheres becomes obliterated, or rather absorbed into the hemi- 

 spheres, and the olfactory nerves from this time arise directly 

 from the hemispheres. During the third day the olfactory 

 pits deepen rapidly, and the distal ends of the olfactory nerves 



