590 ON THE THEORY OF THE VERTEBRATE SKULL 



of the nasal processes, which now proceed from the coalesced part in question, 

 also become but little denser in texture for the present, though they elongate 

 considerably. 



The lateral parts and the upper wall of the cranium, with the exception of the 

 auditory capsules or of the subsequent bony labyrinth, remain merely membranous 

 up to -the ;end of the second period, consisting in fact only of the cutaneous 

 covering, the dura mater, and a Uttle interposed blastema, which is hardly per- 

 ceptible in the upper part, but increases in the lateral walls, towards the base of 

 the skull. 



The chorda vertebralis reaches, in very young embryos of the snake, to 

 between the auditory capsules, and further than this point it can be traced 

 neither in the snake nor in other Vcrtebrata, at any period of life, as manifold 

 investigations, conducted with especial reference to this point, have convinced me. 

 At the beginning of the third period, the basal plate chondrifies, at first 

 leaving the space beneath the middle of the cerebellum membranous ; but this 

 also eventually chondrifies, and is distinguished from the rest of the skull only by 

 its thinness. 



Lateral processes grow out from the basal cartilage just in front of the occipital 

 foramen, and eventually almost meet above. They are the exoccipitals. 



The two lateral trabeculae, parts which I have also seen in frogs, lizards, birds, 

 and mammals, chondrify at the beginning of the third period. At first, they 

 'pass, separate from one another throughout their whole length, as far as the 

 frontal wall, on entering which they come into contact ; are more separate 

 posteriorly than anteriorly, and present, in their mutual position and form, some 

 similarity with the sides of a lyre. But as the eyes increase, become rounder, and 

 project, opposite the middle of the trabeculae, downwards towards the oral cavity, 

 the latter are more and more pressed together, so that even in the third period 

 they come to be almost parallel for the greater part of their length. Anteriorly, 

 however, where they were already, at an earlier period, nearest to one another, 

 they are also pressed together by the olfactory organs (which have developed at 

 their sides to a considerable size), to such a degree, that they come into contact 

 for a great distance and then completely coalesce ; they are now most remote 

 posteriorly, where the pituitary body has passed between them,^ so that they seem 

 . still to embrace it. Anteriorly, between the most anterior regions of the two nasal 

 cavities, they diverge from their coalesced part as two very short, thin, processes 

 or cornua, directed upwards, and simply bent outwards. 



" It has been seen above that the median trabecula does not chondrify, 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 trabecule 

 of the skull a considerable gap, which is only closed by the mucous membrane ot 

 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 anteriorly pointed streak of blastema, which, shortly 

 before the end of the third period, acquires a cartilaginous character, and subse- 

 quently becomes the body of the presphenoid.^ 



^ The pituitary body, however, as Rathke now admits, does not pass between the 

 trabecule, and is developed in quite a different manner from that supposed in the memoir on 

 Coluber. 



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

 embryonic cranium in Gasierosieus and Rana, 



