DfiVELOPMENT OF CRANIAL NERVfiS IN THE CHICK. IS 



Two other points in this figure require notice — 



1. That the whole section consists of epiblast. 



2. That there is no trace of the neural ridge. 



Kg. 3 passes through the posterior part of the optic vesicles 

 {ov), which are very prominent^ though the medullary folds 

 have not yet coalesced. The section, which is a somewhat im- 

 perfect one, owing to the brittleness of the embryo from which 

 it was cut, passes through the fore gut, of which the roof only 

 {/ij/) is seen in the figure. The neural ridge forms a very 

 prominent outgrowth {m) on either side of the summit of the 

 canal. 



rig. 4 passes through the widest part of the mid brain^ i. e. 

 the same spot as fig. 1, but at a slightly later date. Complete 

 coalescence of the medullary folds has occurred, and the 

 external epiblast {ep) now forma a continuous layer across the 

 top of the canal. The neural ridges i^ii) are much more pro- 

 minent than in fig. 1, and are fused and continuous with one 

 another. 



A section taken through the hind brain is given in fig. 5, in 

 which the canal is not yet closed, while the neural ridge {m) 

 forms a conspicuous object on either side. It will be seen that 

 the notochord (w) is represented as in continuity with the hypo- 

 blast of the fore gut {fg). I hope to refer to this point in 

 detail on some subsequent occasion, and mention it here merely 

 to point out that it is so represented purposely, and not through 

 carelessness. In the section figured there is perfect continuity 

 between the two structures. 



On comparison with the preceding stage we find the neural 

 ridge considerably increased, both in longitudinal extent and in 

 actual size at any given point. It now extends from about the 

 middle of the optic vesicles to the posterior part of the hind 

 brain ; its greatest lateral extent is at the point at which it first 

 appeared, i. e. the centre of the mid brain. We notice also that 

 the ridge tends to become specially prominent at certain points, 

 and we shall subsequently find that these prominences are the 

 first definite commencements of individual nerves. Thus, at the 

 slight constriction separating the mid and hind brains the ridge 

 is not so large as it is either opposite the middle of the mid 

 brain (fig. 4), or a short way further back in the hind brain 

 (fig._5). 



Fig. 4 shows, further, that when closure of the neural canal is 

 etfected the neural ridge remains in connection with the neural 

 canal, but separates completely from the external epiblast, so that 

 all trace of the original connection is permanently effaced. 



The extension forward of the neural ridge to the fore brain, as 

 shown in fig. 3, is one of the most remarkable and interesting 



