ON THE THEORY OF THE VERTEBRATE SKULL 593 



chylosed with the anterior edge of the lateral part of the occipital bone, and also 

 forms a small, irregularly-shaped, but longish scale, which contains the deeper or 

 lower part of the posterior crus of that semicircular canal, and besides this, the 

 lower sac, or representative of the cochlea of the auditory labyrinth. The remain- 

 ing bony mass of the auditory cartilage, however, includes the greater part of the 

 membranous portion of the labyrinth, and is the largest. The same phenomenon, 

 viz. that the petrosal bone breaks up, as it were, into three pieces, of which 

 two coalesce with the occipital bone, occurs also, according to my observations, 

 in Lacerta agilis, and probably takes place in like manner, if we may conclude 

 from the later condition of the petrous bone to the earlier, in Crocodilia and 

 Chelonia. A squama temporis and a mastoid are, as I judge, never formed in 

 OphidiaP 



Yet what is the osseous mass which eventually coalesces with the exoccipital 

 but the mastoid ? I have indicated above what I believe to be the true ophidian 

 squamosal. 



VI. — On the development oj the Ossified Vertebral Column. 



The concise statement of the general nature of this process which I have given 

 above, is based, partly on the observations of Vogt, Rathke, and Remak, and 

 partly on my own. As great misunderstanding seems to me to have prevailed on 

 this head, I have put together in the present note, all the most important evidence 

 I have been able to collect on this highly interesting subject, accompanying it with 

 a running commentary. I have done this the more willingly, as the accounts of 

 the mode of development of vertebrae in general, in our own language, which I have 

 met with, are strangely meagre. 



Development of the Spinal Column in Fishes. 



I. Blennius viviparus (Rathke, ' Bildungs- und Entwickelungsgeschichte des 

 Blennius viviparus! 1833). 



The surface of the notochord hardens, and acquires a fibro-membranous con- 

 sistence, while its inner substance becomes glassy and transparent, so that the 

 notochord is separated into sheath and contents, as in the lamprey. A segmenta- 

 tion next takes place in the sheath. " At successive intervals it increases, more 

 and more, in density and solidity, acquires in places almost the constitution of 

 cartilage, and there thus arise a great number of successive, very fine, narrow 

 rings, which are connected by much narrower, far less solid, but also far less 

 transparent and more whitish-coloured parts, like sutures." 



When this segmentation has commenced, "a number of cartilaginous, very 

 short, thin and rod-Uke processes, which run in pairs from each member of the 

 vertebral column, where its upper side passes into the two outer ones, appear, pass 

 upwards in the walls surrounding the spinal marrow, and enclose its lower cords. 

 At first, therefore, each pair of processes are separated by a considerable interval 

 throughout their entire length. Subsequently their upper ends approximate (in- 

 creasing in length, and at the same time accommodating themselves to the curve 

 of the spinal marrow, and bending round it) more and more closely, till they, 

 at last, meet above the spinal marrow, and soon after this has happened, coalesce 

 into an arch. 



" Contemporaneously with these processes, and in the same way, there arise 

 from the vertebral column (though only from its hinder half, or that which con- 

 stitutes the foundation of the tail) a number of other processes similar in form and 

 structure, which spring from the junction of the under with the lateral faces of the 



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