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inner or pharyngeal aspect of the visceral wall. Five, of which the 

 two anterior are the longest and about equal, while the others 

 gradually diminish in length from before backwards, can be distinctly 

 observed. They mark out the boundaries of a corresponding number 

 of " visceral arches," and there is sometimes an appearance as of a 

 sixth visceral arch behind the last cleft. A horizontal section shows 

 that these arches differ in nothing but their relative size in no other 

 respect can one of them be distinguished from the other. 



The anterior visceral cleft lies in a transverse plane, immediately 

 behind the angular bend of the floor of the craniospinal cavity, or, 

 as I shall henceforward term it, mesocephalic flexure. Consequently 

 the posterior part of the first visceral arch passes into the future 

 basis cranii close to the flexure. 



The parts of the cerebrum are now distinguishable. It is bent in 

 correspondence with the mesocephalic flexure, and its most pro- 

 jecting portion, or the angle of the bend, is the rudiment of the 

 mesencephalon. The large rudiment of the pituitary body lies im- 

 mediately in front of the flexure, and is therefore altogether anterior 

 to the end of the notochord and to the posterior part of the first 

 visceral arch. The rudiment of the eye lies at first altogether in 

 front of the flexure, and therefore anterior to the root of the first 

 visceral arch. 



The auditory vesicles make their appearance on each side of a 

 line which would cut the chorda a little behind its anterior termi- 

 nation. They are at first quite free and perfectly distinct from the 

 walls of the cranium, which is in accordance with Remak's state- 

 ment, that they are originally formed by the involution of the epi- 

 dermic layer of the embryo. They long remain separate and easily 

 detachable from the cranial walls. 



Ten days after impregnation, larvae with rudimentary external gills 

 and colourless blood, still exhibited some traces of the mesocephalic 

 flexure, but the angle formed by the anterior and posterior portions 

 of the cranium was very obtuse ; the base of the cranium had, in 

 fact, undergone a gradual straightening. The rudiments of the 

 cranial skeleton had made their appearance, and consisted, behind 

 the mesocephalic angle, of a broad semi-cartilaginous plate enclosing 

 the anterior end of the notochord, but not covering it above or below. 

 It is not as yet adherent to the auditory sacs. 



