48 



Mr. A. Milnes Marshall on the 



[Mar. 8, 



out laterally into paired processes. These processes are the rudiments of 

 the cranial nerves ; the cells composing them are, like those of the ridge 

 itself, small and spherical, and differ markedly from both the elongated 

 cells of the external epiblast, and the large, loosely arranged, branching 

 and irregularly shaped mesoblast cells. 



At forty-three hours the first pair of these processes arises from the 

 anterior part of the hind brain ; it subsequently develops into the fifth 

 nerve. 



Immediately in front of the auditory involution (which at this period 

 is a wide and very shallow pit) a large outgrowth arises on either side, 

 from which the facial and auditory nerves are derived. 



A large outgrowth from the median ridge commences on either side a 

 short distance behind the auditory pit, and is of considerable longitudinal 

 extent, reaching as far back as the middle of the first proto vertebra. 

 Erom this outgrowth are developed the glossopharyngeal nerve and the 

 several branches of the vagus. 



The outgrowth of spherical cells from the summit of the neural 

 canal, forming the longitudinal ridge above alluded to, is not con- 

 lined to the hind brain, but is continued backwards without any break 

 some distance down the spinal cord. In the spinal cord, as in the brain, 

 this ridge gives off at intervals paired lateral processes, which extend 

 outwards just beneath the superficial epiblast. These processes corre- 

 spond in number to the protovertebrse, and are the rudiments of the 

 posterior roots of the spinal nerves. Each process has a longitudinal 

 extension equal to about half a proto vertebra, opposite the posterior 

 part of which it is situated. In the case of the first few spinal nerves 

 the processes are somewhat larger, and extend back so as to overlap the 

 anterior parts of the succeeding protovertebrse. 



This description, it is believed, differs from any previously published 

 account of the development of the nerves in the chick, but agrees re- 

 markably closely with Balfour's * account of the development of the 

 nerves, both cranial and spinal, of Elasmobranchs, and is in accordance 

 with Hensen'sf observations on the development of the posterior roots 

 of the spinal nerves in the rabbit. 



Opposite the centre of each protovertebra the external epiblast grows 

 downwards as a small conical process on either side of the spinal cord 

 and in close contact with it. These processes were mistaken by His£ 

 for the commencements of the spinal nerves, but are clearly seen to have 

 no connexion whatever with the nerve-rudiments. His is the only pre- 

 vious observer who assigns an epiblastic, instead of a mesoblastic, origin 

 to the nerves in the chick ; he, however, derives them directly from th© 



* Phil. Trans, vol. 166, pt. 1. 



t Zeitschrift f Anatomie u. Entwickelungsgeschichte, 1876, Bd. i. 

 \ Die erste Anlage des Wirbelthierleibes. * 



