18 THE VOYAGE OF H.M.S. CHALLENGER. 



acquired its proper Chelonian characters ; it was more like that of a Batrachian in the 

 last stage. 



The roof is open up to the occipital ring (s.o.) ; the sides are continuously 

 cartilaginous {cd.s.,o.s) ; the hind part of the base {h.s.,b.o.) is very deep ; and the 

 fore part of the base arises now from the top of a steep wall, the orbito-nasal septum 

 {p.e.,s.n.). 



This sectional vertical view shows a chondrocranium cjuite similar to that of the 

 chick near the end of the first week of incubation (Phil. Trans., 1869, pi. Ixxxi. fig. 3), 

 for at that time the prognathism of the Fowl's face is only just beginning ; and if the 

 premaxillaries existed then they would be vertical, and not almost horizontal, a direction 

 they possess in the bird very early. 



The composition of the hollow posterior, and vertical anterior, basal parts will be 

 largely illustrated by other views (sectional) ; the fore part of the nasal septum shows a 

 small pre-nasal rod directed downwards; the "alse" or shelving side- walls are well seen 

 in this view. 



As in the embryo Whale' and Pig (Phil. Trans., 1874, pi. xxxiv. fig. 6) the orbito- 

 sphenoids (o.s.) are very large, and a lesser fold of cartilage is continued from them along 

 the sides of the cavity for the mid-brain, and unites with the top of the ear-capsule ; 

 this is the large " alisphenoid " {al.s). 



The orbito-sphenoids are notched by the optic nerve (2) postero-inferiorly, and the 

 alisphenoids similarly by the trigeminal nerve (5) ; these latter " wings " are buttressed 

 on their inner side by the dilated ends of the huge " post-clinoid wall" {'p.cl). 



This " wall " is due to the enfolded form of the embryonic head, but as the para- 

 chordal cartilages stop short above, and do not follow the folding of the mid-brain, they 

 of course only form a single plate thus ending free above. 



We shall see that the median tracts of the basis cranii do, in some degree, 

 become folded over ; but the paked cartilages stop above, and begin again below ; 

 there is, therefore, a hiatus in the chondrocranial floor, which, where it begins its 

 prochordal growth, below and in front, is a sort of added part, or outgrowth of the 

 primary basal tracts, which would, normally, end where the notochord itself ends. 



I look upon this peculiar modification of the basi-cranial axis as due to a specialisa- 

 tion taking place for the purpose of finishing .an actual or practical end to the skull, the 

 organic end having been bent under the head ; its position would be directly below the 

 fore-brain, after the cranial flexure had taken place. 



Nevertheless, I do not doubt the homology of these paired tracts and their lateral 

 alse (trabeculse and orbito-sphenoids) with the paired tracts aiid their ate that grow on 

 each side of the notochord (post-sphenoidal and occipital regions). 



This outgrowing front region of the skull is a correlate of the outgrowing and 



' See Eschricht, On Balcena japonica. Copenliageii, 1869 (pi. ii. flgs. 1-3, Tc). 



