90 Journal of Comparative Neurology. 



vesicles, obviously the tube is dilated in three places and con- 

 tracted in two places. Then shall v/e say that a bending in of 

 the walls, so as to constrict the caliber of the tube, is sufficient 

 to demarcate a segment ? Why do we not have just as much 

 right to say that the opposite condition, or a bending out and 

 consequent widening of the caliber, demarcates a segment? Or 

 may not also a marked change in the character of the whole or 

 of a part of the walls themselves serve to distinguish one 

 segment from another? 



Now if, after development has progressed for a certain time, 

 we carefully examine the third vesicle, we shall find that, — 



A. I. The roof of the cephalic part is composed of sub- 

 stantial nervous parietes. 



2. That later a pari of this roof is clothed with a layer of 

 ectocinerea. 



3. At a point a little caudad of the caudal border of the 

 substantial parietes of the roof is formed a bend (pons flexure) 

 which involves the entire brain tube. The result of this 

 flexure is the formation of a transverse groove which extends 

 clear across the tube. The sides are also forced out ecto-laterad 

 in the form of a U-like fold which may be compared to the 

 rolling collar which the tailor sometimes puts upon garments. 

 As a result of these changes it is obvious that the caliber of the 

 tube is widened at this point. 



4. The substantial parietes are continuous only in the 

 floor of the cavity. Compare Figs. 4, 5, PI. I, and Figs. 49, 

 50, 53. PI. V. 



5. The later growth in this part cephalad of the pons flex- 

 ure is, ia) dorsad, {p) laterad, {c) ventrad. 



B. The part caudad of the flexure is distinguished from 

 the part cephalad by the following characters : 



1. The roof is a thin membranous tela and remains so 

 during the life of the individual. 



2. It never possesses any ectocinerea. 



3. It has no growth dorsad. 



4. Its growth is in a ventral and lateral direction only. 

 From these facts it will appear that while these two re- 



