1^ DR. A. MILNES MARSHALL. 



vesicles ; backwards, along the whole length of the brain and a 

 certain distance down the spinal cord. Its first appearance 

 precedes the closure of the neural canal, but after about the 

 fortieth hour the closure of the canal proceeds backwards more 

 rapidly than the growth of the neural ridge, so that in the 

 greater part of the length of the spinal cord the ridge is deve- 

 loped as an outgrowth from the summit of the cord itself, and 

 never has any connection with the external epiblast. The ridge 

 early attains a great prominence in the region of the mid brain, 

 but shortly afterwards undergoes a great diminution in size, and 

 becomes almost indistinguishable. 



The early appearance of this ridge, its perfect continuity along 

 the whole length of the brain and part of the spinal cord, its 

 rapid growth and almost as rapid shrinking in certain parts, are 

 Very remarkable features from a morphological point of view. 

 Some of these I shall have occasion to discuss further on. One 

 point I may notice here. The perfect continuity of the ridge 

 along the whole length of the brain aj)pears to be a very powerful 

 argument against the existence of any ancestral perforation of 

 the central nervous system by the oesophagus either in the 

 neighbourhood of the fourth ventricle, as suggested by Dr. Dohrn, 

 or at any other point in its length. The morphological signifi- 

 cance of the neural ridge is by no means evident. Balfour 

 describes it as existing in the body and hind brain of Elasmo- 

 branchs,' and remarks concerning it that "there can be little 

 doubt that it is some sort of remnant of an ancestral stucture in 

 the nervous system, and would appear to indicate that the central 

 nervous system must originally have been formed of a median 

 and two lateral strands.''^- 



I have now brought the history of the neural ridge of the 

 chick up to the point at which my previous account commenced. 

 I have already'^ shown how from this ridge the rudiments of 

 certain of the cranial iierves, and in the body the posterior roots 

 of the spinal nerves, are developed ; and have pointed out how 

 exceedingly closely these processes correspond with those occur- 

 ring in Elasmobranchs, as described by Balfour. 



Balfour describes the cranial nerves of Elasmobranchs as being 

 developed successively from before backwards, the first to appear 

 being the fifth nerve. He does not mention any extension of 

 the neural ridge forwards in front of the auditory capsule, an exten- 

 sion that may, how^ever, very possibly occur at an earlier stage than 

 he has noticed as yet. His descriptions do not show whether the 

 ridge makes its first a})pearancc in Elasmobranchs before or 



' 'Journal of Anatomy and Physiology,' vol. xi, part iii, p. 45S. 

 2 Loc. cit., p. 426. 

 ^ Loc. cit. 



