176 
MR. W. K. PARKER ON THE STRUCTURE AND 
(ecto-ethmoidal) mass of cartilage I find a thin but rough irregular layer of bone, just 
where the Teleostei have their proper ecto-ethmoid (Plate 18, figs. 1 and 2, e.eth'.). Also 
over the optic foramen (II.) there is a very large splint (os'.) applied to the orbital 
cartilage: it runs upwards and forwards. On the right side only (fig. 1, al.s.) there 
is a much smaller plate in the alisphenoidal region, and behind and partly round the 
foramen ovale (Y.) a thin semi-annular plate (pro'.) representing the prootic of the 
higher types. But no direct grafting of bone on the cartilage can be seen, and the 
affinity of the bone for the cartilage or, vice versa-, of the cartilage for the bone, is here 
extremely feeble; there is no material interaction ; the co-ordinating force, however, 
has produced a plate of the proper form, and put it ready for use in the proper place. 
Summary and comparison with other types. 
It is evident that we have in the Sturgeon a form which is practically intermediate 
between the Selachians and the Osseous Ganoids (Holostei); the form of the larvrn 
(Plate 12) suggests this view at once. I must again refer the reader to the researches 
of Salensky and Balfour on the embryology of this type; my own recapitulation 
and comparison must be confined to the cephalic skeleton. 
A. The primordial skull. 
We saw that in the larvse of Acipenser ruthenus only about a third of an inch in 
length (9-J millims.) the “ embryonic cartilage” had largely become “ hyaline ; ” that 
the foundations of the cranium were laid, and the visceral arches were differentiated 
and becoming quite solid. Here, it would seem, that in so small a larva of so large 
a Fish—and that Fish lying at the base of the great archaic group of the Ganoids—we 
have a good chance of seeing the primordial vertebrate skull in its utmost simplicity. 
The fact is, that we have a confusingly simple state of things. 
In the fore part of the spine, as well as hi the whole basi-cranial region, the 
paired skeletal tracts that lie right and left of the mesoblastic sheath of the notochord, 
the hardening cartilage shows no signs of segmentation or intercalary vertebral sub¬ 
division ; this is just like what occurs in the Selachians (Trans. Zool. Soc., vol. x., 
plate 35). Moreover, this Chondrosteous Ganoid remains in this condition, as far as I 
can find, throughout life, and does not acquire the occipito-cervical articulation, so well 
known in the Selachians, but which in them is a secondary modification of the parts. 
By careful comparison of all the facts I have been able to gather by observation of 
many types and at many stages, I cannot help comjng to the conclusion that the 
normal intercalary vertebral segmentation with which we are so familiar in the post- 
cephalic region of the Vertebrata, generally, is a comparatively late and secondary 
specialization in the evolution of this, the highest, group of animals. 
If Mr. Balfour’s suggestion (‘ Comp. Embryol.,’ vol. ii., p. 366) be true, viz. : that 
the fore-brain with its special optic and olfactory outgrowths is a sort of outgrowth or 
