164 Journal of Comparative Neurology. 



phological front of the head. One cannot carefully trace the 

 genesis of the brain and head in Ophidia without being convinc- 

 ed that this is the clue to the complicated problem of cepha- 

 logenesis. The query so often raised as to the reason for the 

 restriction of the chorda to prae-infundibular regions of the head 

 requires no further discussion if it is recognized that the pituit- 

 ary marks the beginning of a dorsal surface and all structures in 

 the skull arising cephalad of it cannot be considered as segmen- 

 tal in a strict sense. The stomatodeum is that portion of the 

 ectoderm which has been phylogenetically interpolated between 

 the primitive subterminal mouth and the present oral aperture. 

 (See Plate XIX, Fig. i.) The structure of epithelium and the 

 origin of the neuroblasts within it at the ofactory area is well 

 shown in plate XV, Fig. 8, the portion forming the lense and 

 its capsule at plate XV, Fig. 7. 



The Cranial Nerves. — In E II, the trigeminal is already 

 quite well developed. The vesicle is moderately expanded and 

 upon its lateral aspects a small cluster of cells is seen which, 

 however, does not directly communicate with the medulary wall 

 itself. On the other hand, the connection is very direct and 

 obvious with the seat on the ectoderm at the base of the man- 

 dibular anlag. The nerve at this stage has all the characteristics 

 of a sensory nerve arising from a special centre in the skin. 



In E. II, the three divisions are well distinguished. The 

 ophthalmic extends only as far as to a point dorso-mesad of the 

 eye, while the combined maxiliary and mandiblar rami extend 

 latero ventrad to the angle of the mouth, the former turning rap- 

 idly cephalo-laterad and reaching almost to the eye, while the 

 later dips into the rapidly growing cells of the mandiblar anlag. 

 The vesicle (caudad neuromer) is without an obvious nerve at 

 this stage, although its neuromer is as large as any of the follow- 

 ing ones. It would appear that its dorsal roots enter with the 

 fifth. The seventh nerve passes directly cephalo-ventrad to the 

 hyoid arch. It arises with the auditory. The motor cells lie on 

 the ventro-lateral aspects but do not emerge there. The audi- 

 tory ganglion arises from a special anlag formed with the cap- 

 sule and for a long time does not communicate with the medulla. 



Two roots arising from two neuromers caudad represent 

 the ninth and tenth anlags, the former simply related to the gill 



