ON THE THEORY OF THE VERTEBRATE SKULL S9I 



" Altogether anteriorly, however, where the two trabeculse have coalesced, 

 there grows out of this part, from the two cornua in which it ends, a pair of very 

 dehcate cartilaginous plates. At the end of the third period both plates acquire a 

 not inconsiderable size, take the form of two irregularly formed triangles, and are 

 moderately convex abo\'e, concave below, so as to be on the whole, shell-shaped. 

 The nasal bones are developed upon these, while below them are the nasal cavities, 

 and the nasal glands with their bony capsules. 



" The ate or lateral parts of the two sphenoids do not grow like the lateral 

 parts of the occipital bone out of the basis cranii, whose foundation is formed by 

 .the cephalic part of the chorda, but are formed separately from it, although close 

 to it, in the, ufttil then, membranous part of the walls of the cranium. 



" The ate of the presphenoid (orbitosphenoids), which are observable not very 

 long before the termination of the third period, appear as two truly cartilaginous 

 (though they never redden), irregular, oblong, plates of moderate thickness, lie in 

 front of the optic foramina, at the sides of the lateral trabecute of the skull, 

 .ascend from them upwards and outwards, and are somewhat convex on the side 

 turned to the brain, somewhat concave on the other. The ate magns (ali- 

 sphenoids) are perceptible a little earlier than these. They are formed between 

 the eye and the ear, and also originally consist of a colourless cartilaginous 

 substance ; they appear at the end of the third period as irregular four-sided 

 plates, lie at both sides of the anterior half of the investing plate of the chorda, 

 ascend less abruptly than the ate orbitales, and are convex externally, internally 

 ■concave. 



" The upper posterior angle of each elongates, very early, into a process, which 

 grows for a certain distance backwards, along the upper edge of the auditory 

 • capsule, and appUes itself closely thereto. 



" The auditory capsules, or the future petrous bones,i chondrify, as it would 

 .appear, the earliest of all parts of the skull : the fenestra ovalis arises in them by 

 resorption. 



" The ossification of the snake's skull commences in the basioccipital, or at any 

 rate this is one of the first parts to ossify. At a little distance from the occipital 

 foramen, there arises a very small semilunar bony plate, whose concave edge 

 or excavation is directed forwards ; thereupon the bony substance shoots from 

 this edge further and further forwards, until at length the bony plate has the form 

 ■of the ace of hearts. Its base borders the fontanelle in the base of the skull, 

 which lies under the anterior half of the third cerebral vesicle, while its point 

 is contiguous to the occipital foramen ; for the most part it is very thin, and 

 only its axis (and next to this its whole posterior margin) is distinguished by a 

 greater thickness. The cephalic part of the chorda can be recognized in the 

 axis of this bony plate up to the following period. It passes from the posterior 

 to the anterior end of the bony plate, where it is lost, and is so invested by the 

 bony substance of the plate, that a smaller portion of the latter lies on the upper 

 side of the chorda, a larger portion beneath it. On this account it forms, on the 

 upper side of the plate, a longitudinal ridge, which subsequently becomes im- 

 perceptible by the aggregation of matter at the sides. On one occasion, however, 

 I saw in an embryo which was almost at term, a similarly formed and sized bony 



■ cone, 'which, through almost its entire length, appeared merely to lie on the body 



■ of the basioccipital, since it had only coalesced with it below." 



The nucleus and sheath of the cephalic part of the chorda become gradually 

 broken up and the last trace of them eradicated, as the ossification of the basi- 



1 It will be found from Rathke's statements, further on, that the future petrous bone only 

 .represents a portion of each auditory capsule. 



