DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 39 



and vagus I regard as a secondarily acquired feature, which 

 possesses some interest from the fact that both the anterior and 

 posterior roots of the spinal nerves of the chick are similarly 

 multiple. As far as I have been able to determine, these roots, 

 which vary considerably in size, are not constant in number. 

 They appear to increase in number and complexity with age. 



The cranial nerves and the posterior roots of the spinal 

 nerves arise as outgrowths of the continuous longitudinal 

 neural ridge. I propose, in conclusion, to give the history, as 

 far as I have succeeded in determining it, of the portions of the 

 neural ridge that intervene between successive pairs of nerves. 



Fig. 27 represents a horizontal section through the cervical 

 region of a ninety-three hours' chick close to the dorsal surface; it 

 shows the muscle-plates, of which that marked (^mp) is the 

 second. It passes through the spinal cord, and just touches the 

 hind brain. The cord is slightly constricted opposite the centres 

 of the protovertebrse, and slightly dilated opposite the intervals 

 between successive protovertebrse. 



On the right side the section passes through the point of 

 attachment ot the posterior root of the third sph)al nerve (sp) to 

 the cord ; the nerve is s&en to be ganglionic, and is attached to 

 the cord by several roots, not shown in the figure ; it lies oppo- 

 site the anterior half of the corresponding muscle-plate.^ The 

 ganglion (*/j) is connected by a nervous commissure (com) with 

 the ganglion of the nerve in front of it. 



In fig. 28 these relations are still better seen. The section is 

 taken from the same embryo as fig. 27, but a little further back, 

 passing through the fifth, sixth, and seventh protovertebrse. 

 Sp is the ganglion of the seventh cervical nerve on the left side, 

 at the point of attachment of the posterior root to the cord. 

 The commissure {com) is well seen ; it is dilated opposite the 

 anterior halves of the protovertebrae. 



The posterior roots of all the spinal nerves are connected 

 together in this way by a longitudinal commissure. At the stage 

 figured the nerves have acquired their secondary attachment to 

 the sides of the cord. From the accounts I have already given 

 there can, I think, be no doubt that the commissure is formed 

 from the intervening portions of the neural ridge between suc- 

 cessive pairs of spinal nerves. 



This commissure was first described by Balfour^ who justly 



' In my former paper I erroneously stated that the posterior roots are 

 situated opposite the posterior halves of the corresponding protovertebrae. 

 Figs. 24, 27, and 28 show conclusively that they He opposite the anterior 

 halves. 



2 ' Phil. Trans.,' vol. 166, part i, and ' Journal of Anat.,' loc. cit., p. 424. 



