439 



but eventually disappears ; in its place, a truly cartilaginous short thick 

 band grows into the fold of dura mater from the cartilaginous basal plate. 



"Where the pituitary gland lies, there remains between the lateral 

 trabeculss of the skull a considerable gap, which is only closed by the 

 mucous membrane of the mouth and the dura mater. But there arises in 

 front of this gap, between the two trabeculae, as far as the point where 

 they have already coalesced, a very narrow, moderately thick, and ante- 

 riorly pointed streak of blastema, which, shortly before the end of the 

 third period, acquires a cartilaginous character, and subsequently becomes 

 the body of the presphenoid*. 



" Altogether anteriorly, however, where the two trabeculse have coa- 

 lesced, there grows out of this part, from the two cornua in which it ends, 

 a pair of very delicate cartilaginous plates. At the end of the third period 

 both plates acquire a not inconsiderable size, take the form of two irre- 

 gularly formed triangles, and are moderately convex above, 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 alss or lateral parts of the two sphenoids do not grow like the 

 lateral parts of the occipital bone out of the basis cranii, whose founda- 

 tion is formed by the cephalic part of the chorda, but are formed sepa- 

 rately from it, although close to it, in the, until then, membranous part 

 of the walls of the cranium. 



" The alse 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 mo- 

 derate thickness, lie in front of the optic foramina, at the sides of the 

 lateral trabeculae of the skull, ascend from them upwards and outwards, 

 and are somewhat convex on the side turned to the brain, somewhat con- 

 cave on the other. The a]ae magnge (alisphenoids) 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 j 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 alse 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 applies itself closely thereto. 



"The auditory capsules, or the future petrous bones f, 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 



* Compare with these statements, the figures and descriptions given above of 

 the embryonic cranium in Gasterosteus and Rana. 



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

 bone only represents a portion of each auditory capsule. 



