436 



stretched upon a fibrocartilaginous frame, which is ordinarily attached to 

 the squamosal, exoccipital, basisphenoid, and quadratum. In many gal- 

 linaceous 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. 



IV. On the modifications 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 pri- 

 mary 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 ob- 

 servations 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 condition 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 forma- 

 tion of the vertebrae 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 trabeculae at their first appearance formed two narrow 

 and not very thick bands, which consisted of the same gelatinous sub- 

 stance as that which constituted 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 some- 

 what 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 



* Entwickelungsgeschichte der Natter, 1839. 



