ON THE THEORY OF THE VERTEBRATE SKULL 575: 



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 chords a little behind its anterior termina- 

 tion. They are at first quite free and perfectly distinct from the walls, 

 of the cranium, which is in accordance with Remak's statement, that 

 they are originally formed by the involution of the epidermic 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. 



That part of the middle of the basis cranii which underlies the 

 pituitary body is not converted into cartilage, but remains membranous, , 

 and may be called the " subpituitary membrane." The delicacy of" 

 this membrane is so great that it is easily torn, when the pituitary 

 body seems, as Rathke originally supposed, to unite with the palatine 

 mucous membrane. But that this is not really the case, is readily 

 demonstrable in an embryo whose tissues have been sufficiently 

 hardened with alcohol or nitric acid. 



The cartilaginous basal plate gives off a prolongation on either 

 side of the subpituitary membrane. This, the " cranial trabecula " 

 (Schadelbalke of Rathke), passes forwards with a slight convexity 

 outwards, and then turning inwards comes into contact with its 

 fellow (from which, however, it is at first distinct), and spreads out 

 into a broad, flat, elongated process, which I shall term the ethmo- 

 vomerine cartilage. 



Behind the eye and just in front of the auditory capsule (in the 

 posterior part of the first visceral arch, therefore), a cartilaginous 

 process lies, which is connected proximally with the root of the 



