1877.] Development of the Nerves in the Chick. 



49 



external epiblast, while, accordiDg to the description just given, they 

 really arise from the involuted epiblast of the neural canal. 



From their mode of origin the cranial and the anterior spinal nerves 

 will be seen to be all connected together at first by a longitudinal 

 commissure of spherical cells, while the two nerves of each pair, whether 

 cranial or spinal, are also connected together across the top of the 

 neural canal. 



The attachment of the nerve, whether cranial or spinal, is at first to 

 the extreme summit of the neural canal. Shortly after their appearance 

 the attachments shift slightly outwards, and, in the case of the spinal 

 nerve, become much more slender. This shifting is believed to be 

 apparent rather than real, and to be caused, as first suggested by Bal- 

 four, by rapid growth of the cells at the summit of the canal, which 

 has the effect of separating the roots of the two sides from one another 

 and forcing them apart. 



Though the proximal part of the nerve-root becomes thus more slender 

 in the spinal nerves, the distal part enlarges considerably, and grows down 

 as an oval mass (the spinal ganglion) between the spinal cord and the 

 proto vertebrae. At this period the most prominent part is situated 

 opposite the interval between two protovertebrse. 



During the third day a great change occurs in the point of attachment, 

 which is now considerably lower down, in the position occupied by the 

 root in the adult. The nerve is now attached, not by its apex, but by a 

 small process growing out from its side, and projects considerably above 

 the point of attachment. Owing to the surrounding mesoblast this stage 

 is very difficult to investigate ; but the appearances strongly suggest that 

 the original attachment of the nerve to the summit of the cord is lost, 

 and a new one acquired lower down, and that the projection of the nerve 

 above the point of attachment, which becomes inconspicuous very shortly 

 afterwards, is a remnant of the original attachment. 



The anterior roots of the spinal nerves arise later than the posterior, 

 and have not been observed earlier than the latter part of the third day. 

 They appear as small outgrowths from the lower part of the sides of the 

 spinal cord, and from the first occupy the position held by them in the 

 adult. This position is indicated before the actual appearance of the 

 roots by a slight convergence of the cells at the outer part of the cord. 

 The anterior roots are very slender, and consist of much elongated cells, 

 contrasting strongly with the spherical or oval cells of the posterior 

 roots. 



Early on the fourth clay each anterior root consists of a number of 

 such processes placed one behind the other, and lying opposite the anterior 

 half of a proto vertebra. The total length of attachment of an anterior 

 root on the fourth day is equal to about half a protovertebra. 



The anterior roots grow outwards, and early in the fourth day join 

 with the posterior roots to constitute the spinal nerves. 



VOL. XXVI. E 



