318 I>R. A. MILNES MARSBALL. 



1 have nothing to add to the arguments already given, 

 though I am fully aware that the point is not yet proved. 

 In the Elasmobranchs, the only other vertebrates in which 

 the presence of a neural crest has been accurately described,* 

 the anterior limits of this crest have not been fixed with 

 certainty. 



3. The shifting down of their roots of origin, one of the 

 most striking features of the segmental nerves, is a very 

 constant and well-marked point in the development of the 

 olfactory nerves. It is well shown for birds in figs. 10 and 

 11, and for the dogfish in figs. 19 and 20. In the dogfish 

 the displacement of the roots is less extensive than in the 

 chick — a point obviously correlated with the greater develop- 

 ment of the cerebral hemispheres in the latter. 



4. The course of the segmental nerves in their early stages 

 is, speaking within certain limits, at right angles to the 

 longitudinal axis of the head at their point of origin. The 

 facial and the postauditory nerves arise from a part of the 

 head in which this axis is a straight line ; the nerves conse- 

 quently run parallel to one another, as is seen in figs. 4 

 and 6. In front of the origin of the facial nerve the axis of 

 the head is, owing to cranial flexure, no longer a straight 

 line, but a curved one. The trigeminal nerve is disturbed 

 only to a very slight extent, but it will be seen in fig. 4 that 

 instead of running parallel to the facial, the two nerves con- 

 verge slightly towards their distal ends. In the region of 

 the midbrain the effects of cranial flexure are very well 

 marked ; but fig. 6 shows that the course of the third nerve, 

 the segmental nerve arising from the midbrain, is still at 

 right angles to the longitudinal axis at its point of origin. 

 Since the direction of the axis at this point is almost at 

 right angles to its original direction, so also the third nerve 

 is seen to take a course almost at right angles to the facial or 

 glosso-pharyngeal. Similarly, the course of the olfactory 

 nerve is at right angles to the longitudinal axis of the head 

 at its point of origin; and its direction is such that if 

 cranial flexure were corrected and the head straightened 

 out the olfactory nerve would run parallel to the third, trigemi- 



accept this explanation. My specimens leave no room for doubt that 

 whatever may be its morphological importance, the neural crest is a per- 

 fectly continuous structure, extending in the chick at the twenty-ninth 

 hour from the anterior end of the optic vesicles nearly to the end of tiie 

 hindbrain. 1 am acquainted with folds such as Prof. KoUiker describes, 

 but have only met with tliem in imperfectly-prepared specimens, and espe- 

 cially in specimens hardened in chromic acid, which, in my hands, at least. 

 Las always proved a peculiarly unreliable hardening agent. 

 ' Ballour, ' Elasmobraiich Fishes,' jip. I'Jl, l'J2, 



