;S88 ON THE THEORY OF THE VERTEBRATE SKULL 



•the interior of the skull causes the alisphenoid to pass behind the exit of the 

 trigeminal, this retrogression ought to have taken place in the ostrich. Nothing 

 of the kind has occurred, however, the trigeminal foramen being a "■ trou de con- 

 Jugaisoti ' between the alisphenoid and the petrosal. It does not even traverse the 

 middle of the alisphenoid, as in the sheep. 



It is unnecessary to discuss the effect of the disappearance of the petrosal, as I 

 have endeavoured to prove that it does not disappear in the lower Vertebrata. 



III. — Connexions of the tympanic memh-ane i7i Birds. 



According to Kostlin (/. c. p. 216), the tympanic membrane of birds is 

 stretched upon a fibrocartilaginous frame, which is ordinarily attached to the 

 squamosal, exoccipital, basisphenoid, and quadratum. In many gallinaceous birds 

 this frame does not come into contact with the quadratum at all. From these 

 ■circumstances, and from the fact that the quadratum of birds articulates with the 

 ■lower jaw and the jugal arch, which is never the case with the tympanic of 

 mammals, Kostlin concludes, with great justice, that the quadratum is not the 

 -homologue of the mammalian tympanic. 



V\. — On the modificatio7is of the palatosuspensorial arch in Fishes. 



I have very briefly stated my views on this subject in the Quarterly Journal 

 of Microscopical Science for October 1858, hoping at that time to enter more 

 largely upon the subject in this place. But the present Lecture and its notes 

 already occupy so much space, that I must reserve a full statement of what 

 I have to say respecting the palatosuspensorial apparatus of fishes for a future 

 occasion. 



V. — On the development of the Cranium. 



In confirmation of the views which I have adopted, as to the primary uniformity 

 •of plan of all vertebrate crania, I subjoin an abstract of Rathke's most valuable 

 account of the development of the skull in Coluber natrix^ which contains much 

 incidental information relating to the development of the skull in Vertebrata in 

 general. Vogt's obser\-ations on Coregonus and Alytes, and my own on Gasterosteus, 

 Rana and Triton, are in entire accordance with those of Rathke, so far as the 

 primitive structure of the basis cranii is concerned. 



The differences between the basis of the skull and the vertebral column in the 

 ■earliest embryonic conditions are, — 



1. That round that part of the chorda which belongs to the head, more of the 

 blastema, that is to be applied, in the spinal column, to the formation of the 

 vertebrffi and their different ligaments, is aggregated than around the rest of its 

 •extent, and — 



2. That this mass grows out beyond the chorda to form the cranial trabeculae. 

 The lateral trabecular at their first appearance formed two narrow and not very 



thick bands, which consisted of the same gelatinous substance as that which con- 

 stituted the whole investment of the chorda, 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 



Entwickelungsgeschichte der Natter,' 1839 



