RATHKE ON THE DEVELOPMENT OE THE CRANIUM. 
243 
a short time before the end of foetal life with the basisphenoid, which 
grows somewhat towards it, and then represents a moderately thick, 
tolerably long spine, provided at its upper end with a groove, pro¬ 
ceeding from the basisphenoid, and aiding it to close inferiorly the gap, 
which previously lay under the pituitary body. It should be espe¬ 
cially noted that the presphenoid of the Chick is developed from a 
thin lamella of bone, which plainly arises between the primarily 
cartilaginous orbital septum (produced by the coalescence of the 
trabeculae) and its perichondrium. In the Chick, as in Snakes and 
Lizards, the body of the basisphenoid is formed in the investing mass 
just in front of the notochord. 
The basioccipital also arises in the same way as in these Reptiles. 
At the time when the cephalic part of the notochord still persists, 
bone is deposited around this part in the investing mass, and then 
from this point ossification proceeds right and left. 
In Mammals, according to my examinations of Ruminants and 
Pigs, the basioccipital is formed in the region in which the cephalic 
part of the notochord lies; yet only after this has disappeared. 
The basisphenoid arises, to judge by the adjacent parts, in the point 
where, at an earlier period, the anterior part of that plate which is 
formed by the investing mass lay. 
A special presphenoid, however, appears not to be formed in 
Ruminants and Pigs; for I have been able to observe none in these 
animals, even when all the four already ossified alse of the sphenoid 
and the body of the basioccipital possessed a considerable size, and 
the vomer reached almost to the basisphenoid. 
In new-born moles, also, I have observed no presphenoid. 
Kerckringius was led to the same result by his investigations into 
the development of the human skeleton and in Nicolai’s work on 
the same subject (‘ Beschreibung der Knochen des Foetus’) only one 
body of the sphenoid is spoken of. In Ox embryos, with heads 
15 to 18 lines long, and in which each orbitosphenoid already pos¬ 
sessed two large roots embracing the optic nerve, both bones came 
into contact by these roots, and the basisphenoid, which lay between 
the alisphenoids, pressed close against the latter. I am therefore led 
to believe, that the whole presphenoid of these Mammals arises by 
the coalescence of the roots of the orbitosphenoids, partly with one 
another, partly with that piece of bone which lies behind them and 
between the alisphenoids, and that this bone then grows further and 
further forwards below them. In new-born Kittens, on the other 
hand, I saw a special presphenoid quite distinctly.! A similar 
bone seems to occur in many other Mammalia, so far as I could 
make out from their fully developed skulls—viz. in the Seals, 
* Osteogenia foetuum. 
f Whether it had arisen between the trabeculse cranii or out of their substance 
could no longer be made out. 
R 2 
