240 ON THE STRUCTURE OF THE SKULL. 



The tsvo lateral trabecular — parts which I have also seen in 

 Frogs, Lizards, Birds, and Mammals — chondrify at the beginning 

 of the third period. At first, they pass, distinct from one 

 another throughout their whole length, as far as the frontal 

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

 separate posteriorly than anteriorly, and they present, in their 

 relative 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 trabecular, 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, where 

 they w^ere 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 trabecular of the skull a considerable gap, wdrich 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 

 trabecular, 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 subsequently becomes the 

 body of the presphenoid. 



* The pituitary body, however, as Rathke has since admitted, does not pass 

 between the trabecular, and is developed in quite a different manner from that 

 supposed in the memoir on Coluber. 



