DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 15 



We find, then, that at twenty-nine hours the neural canal is closed 

 along the posterior part of the fore brain, along the whole length 

 of the mid brain, and along the most anterior part of the hind 

 brain. The neural ridge does not extend quite to the anterior 

 extremity of the brain. The first few sections show no trace of 

 it, and closely resemble fig. 3 ; it commences, however, in front 

 of the optic vesicles, and extends backwards nearly to the end of 

 the hind brain. It is exceedingly prominent along the whole 

 length of the mid brain, and is nearly as large at the constriction 

 separating the mid from the fore brain, as it is opposite the 

 widest part of the mid brain. The last point to be noticed is 

 that the mode of development of the ridge is precisely the same 

 as it was at its earliest appearance. This is obvious on com- 

 paring fig. 10 taken through the hind brain with fig. 1, which 

 passes through the mid brain of a chick about seven hours 

 younger. 



The next stage I propose to consider is furnished by a forty- 

 three hours' chick, and constitutes the earliest described in my 

 former paper. As I have already given a detailed description, 

 with figures, of this stage,^ I shall only notice it very briefly here. 



The neural canal is closed along the whole length of the brain, 

 and along a portion of the spinal cord corresponding to the 

 anterior three or four protovertebrse. 



The neural ridge is still recognisable on the summit of the 

 optic vesicles, but is not so conspicuous as before. Along the 

 mid brain it is very much reduced in size, and appears in trans- 

 verse sections as a very thin rod of cells, still connected with the 

 summit of the canal. It is very slender, and in some specimens 

 I have found it impossible to distinguish it satisfactorily from 

 the surrounding mesoblast cells, which are smaller and more 

 closely compacted than in the preceding stage. At the constric- 

 tions separating the mid brain from the fore and hind brains the 

 ridge has apparently completely disappeared. In the anterior 

 part of the hind-brain it is present, but is much more slender 

 than before. Further back it again becomes very conspicuous, 

 but is no longer uniform, presenting a strong development just 

 in front of the auditory pit, and another just behind it, while in 

 the intervening part it is much smaller. It also extends a short 

 distance down the spinal cord. 



To recapitulate. About the twenty-second hour a small out- 

 growth of cells appears along the mid brain on each side, at the 

 angle between the external epiblast and the neural canal — the 

 neural ridge. This rapidly extends both forwards and back- 

 wards : forwards, as far as the anterior part of the optic 

 » Loc. cit., pp. 491—497, and Plate XX, figs. 1—6, 



