THE SKULLS OF REPTILIA AND AVES. 237 



occipital and the squamosal, is a cleft which leads to the occipital 

 surface in the drv skull. 



The early stages of the development of the skull of a Bird 

 have already been described. The process of formation of the 

 Reptilian skull has been admirably worked out by Rathke in the 

 Common Snake, Coluber natrix, and I conclude this Lecture by 

 an abstract of his researches on this subject.* 



The differences between the basis of the skull and the verte- 

 bral column in the earliest embryonic condition are, — 



1 . That round that part of the notochord which belongs to the 

 head, more of the blastema, that is to be applied, in the spinal 

 column, to the formation of the vertebrao and their different 

 ligaments, is aggregated than around the rest of its extent, and — 



2. That this mass grows out beyond the notochord to form 

 the cranial trabecule. 



The lateral trabecule, at their first appearance, formed two 

 narrow and not very thick bands, which consisted of the same 

 gelatinous substance as that which constituted the whole invest- 

 ment of the notochord, and were not sharply defined from the 

 substance which lay between them and at their sides, but seemed 

 only to be two thickened and somewhat more solid, or denser, 

 parts of that half of the basis of the cranium, which lies under 

 the anterior cerebral vesicle. 



Posteriorly, at their origin, they were separated by only a 

 small interval, equivalent to the breadth of the median trabecula, 

 and thence swept in an arch to about the middle of their length, 

 separating as they passed forwards ; afterwards they converged, 

 so that, at their extremities, they were separated by a very 

 small space, or even came into contact. Altogether they formed, 

 as it were, two horns, into which the investing mass of the noto- 

 chord Avas continued forwards. The elongated space between 

 them, moderately wide in the middle, was occupied by a layer 

 of softer formative substance, which was very thin posteriorly, 

 but somewhat thicker anteriorly. Upon this layer rested the 

 infundibulum ; and in front of it, partly on this layer, partly on 

 the trabecule, that division of the brain whence the optic nerves 



* Entwickeluugsgescliiclite der Natter, 1839. 



