262 . THE CHICK. 



(ii) The non-ganglionated nerves. These arise as direct out- 

 growths from the nerve cells of the brain or spinal cord. The 

 nerves of this category develop at a rather later period than 

 those of the former one ; they are all motor in function, and to 

 them belong the sixth, and perhaps some of the other cranial 

 nerves, and the ventral or motor roots of the spinal nerves. 



Certain of the cranial nerves cannot at present be referred 

 with certainty to either category ; but in their cases our know- 

 ledge of the developmental history is incomplete, and further 

 research is necessary before any definite statement can be made 

 concerning their real nature. 



With regard to the nerves definitely included in the first 

 category, a distinction must be made between the cranial and 

 the spinal nerves, similar to that already described in the frog. 

 The cranial nerves, in their growth outwards, lie at first very 

 superficially, just beneath the external epiblast. Near their 

 distal ends they early acquire connection with localised thick- 

 enings of the external epiblast, situated about the horizontal 

 level of the notochord, and just above the dorsal borders of the 

 gill-clefts. Prom these thickened patches of epiblast, which are 

 probably to be regarded as sense organs, cells are budded ofi" 

 into the nerves, which appear to take a direct part in their 

 further development. 



The spinal nerves, on the other hand, are from the first 

 more deeply situated. They lie between the spinal cord and the 

 muscle plates (Fig! 124, ne), and do not acquire the connec- 

 tions with the external epiblast which are characteristic of the 

 cranial nerves. 



b. The Cranial Nerves. The first trace of the cranial nerves 

 appears, in the chick, in the region of the mid-brain, about the 

 twenty-second hour. At this stage, slightly younger than that 

 shown in Figs. 110 and 118, the neural folds have nearly met, in 

 the region of the head and neck, but have not yet coalesced at 

 any part of their length ; while in the body region the central 

 nervous system is still a widely open groove. Only one or two 

 pairs of mesoblastic somites are as yet present. 



At the lips of the neural groove there is on either side a 

 ridge-like outgrowth of epiblast cells, from the angle between 

 the external epiblast and the wall of the neural canal. This 



