13 DR. A. MILNES MARSHALL. 



spicuous dilatation, separated by a slight, but evident constric- 

 tion from the middle vesicle, M-hich is separated by a less marked 

 constriction from the posterior vesicle ; and before closure of 

 the anterior vesicle is effected the optic vesicles are already 

 prominent objects. Coalescence of the medullary folds occurs at 

 these constrictions before it is effected in the dilated vesicles 

 themselves. The point is one of very minor importance, but 

 it was necessary to give this explanation in order to justify the 

 language used in the preceding description. 



Thus, the earliest indication of nerves in the chick occurs 

 about the twenty- second hour in the form of a longitudinal ridge 

 of spherical cells on either side at the angle of inflection of the 

 epiblast to form the neural canal. This ridge is but slightly 

 marked, and extends along the mid brain, and a short distance 

 down the hind brain, being most prominent opposite the widest 

 part of the mid brain. The points of chief interest appear to be — 



1. The neural ridge appears before closure of the neural canal 

 is effected, so that the ridges of the two sides are primitively 

 independent of each other. 



3. The ridge is not developed directly from the external epi- 

 blast or from the iieural canal, but from the re-entering angle 

 between the two. 



3. The ridge appears first in the mid brain. 



The next stage I propose to consider is illustrated by Plate 

 II, figs. 2, 3, 4, and 5, representing transverse sections 

 through various parts of the brain of a twenty-four hours' chick 

 embryo. 



rig. 2 passes through the anterior part of the fore brain 

 [fb). The section appears at first sight somewhat anomalous, 

 inasmuch as it consists of two halves completely separated from 

 one another. This may be due simply to projection forwards 

 of the anterior lips of the medullary folds, but, I believe, is 

 really to be attributed to cranial flexure, which, instead of com- 

 mencing "towards the end of the second day,"^ really begins 

 before the neural canal is closed in. Cranial flexure is prac- 

 tically due to hypertrophy of the dorsal wall of the neural canal, 

 which, during the lirst few days, grows much faster than the 

 ventral wall, so causing the well-known flexure. If this rela- 

 tively rapid growth commences before closure of the canal is 

 efl'ected — and we have already seen that there is hypertrophy of 

 the anterior end of the neural canal from its very earliest appear- 

 ance — fig. 2 becomes at once intelligible ; and it is seen that the 

 lower part of the figure, as well as the upper, consists of the 

 ^s yet unclosed lips of the neural canal. 



' Foster and Balfour, op. cit., p. 7S. 



