50 On the Development of the Nerves in the Chick. [Mar. 8, 



In the cranial nerves no anterior roots have been observed ; but as 

 the observations have not been carried beyond the fourth day, and certain 

 of the cranial nerves have not been observed at all, no conclusion as to 

 their non-existence is to be drawn from this fact, which can only be 

 considered a doubtful confirmation of Balfour's failure to discover ante- 

 rior cranial roots in Elasmobranchs *. 



The facial and auditory nerves have been seen to arise as a single out- 

 growth just in front of the ear ; this speedily divides into an anterior 

 part, which runs downwards in front of the auditory vesicle and becomes 

 the facial nerve, and a posterior part, which is closely applied to the 

 anterior wall of the auditory vesicle and becomes the auditory nerve. 



The fifth nerve arises as a single outgrowth on either side, the posi- 

 tion of which is very constant. The so-called "hind brain" consists 

 at forty-three hours of an apparently variable number of dilatations 

 separated by slight constrictions, and gradually decreasing in size from 

 before backwards. These dilatations are well known, but appear to possess 

 more constancy than is usually ascribed to them ; the most anterior of 

 them is but little smaller than the mid brain. Prom it the fifth nerve 

 arises in all the specimens examined. 



The third, fourth, and sixth nerves have not been observed ; but a 

 slight outgrowth from the summit of the mid brain, noticed in two 

 specimens only, may prove to be the commencement of the third or 

 fourth. 



The olfactory nerves arise towards the end of the third day as solid 

 outgrowths from the anterior end of the fore brain, close to the median 

 dorsal line, and exactly correspond in mode of development and in ap- 

 pearance with the other cranial nerves and with the posterior roots of 

 the spinal nerves. They arise at a time when a section through the 

 anterior part of the fore brain transverse to its longitudinal axis, and 

 passing through the olfactory pits and nerves, is almost perfectly circu- 

 lar in outline, and must therefore be described as arising from the fore 

 brain itself, and not from the cerebral hemispheres, with which they have 

 no connexion at first, and which are not nearly such prominent objects 

 at the end of the third day as they are often described to be. There is no 

 trace of an " olfactory vesicle " in the early stages. 



This mode of development of the olfactory nerve in the chick would 

 seem to be of considerable morphological importance, since, if confirmed, 

 any arguments concerning the composition of the skull, based on the 

 distribution of the cranial nerves, would have in future to take the 

 olfactory nerves into consideration. 



* Loc. tit, p. 189 



