714 
MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODON FOLIUM. 
sents the well-ossified ribs and transverse processes of the Sturgeon. Behind the 
anus the ridges converge slightly, and together with the well-ossified inferior caudal 
arches which are attached to them, form the canal in which the caudal vein is 
contained. 
A transverse section of the vertebral column in the caudal region would disclose 
two canals — one immediately below the notochord, and containing the dorsal aorta ; 
and a second ventrad of the first, containing, as previously mentioned, the caudal vein. 
In comparing such a section with the transverse section through the tail of an embryo 
Scyllium, given by Balfour (‘Journal of Anatomy,’ vol. xi.), it is evident that if 
chondrification were to affect the cells which lie between the aorta and the caudal 
vein, two precisely similar cartilage-bounded canals would be formed. Wedged in 
between the bases of each two adjacent haemal arches, there is a small piece of carti- 
lage exactly corresponding in position below the notochord to the intercalated cartilages 
above. These are interhaemal cartilages, analogous to similarly-situated structures 
described by Gegenbaur* as existing in the Sturgeon. 
But perhaps the most remarkable structure in connexion with the vertebral column 
is a stout, flattened, fibrous band which is attached to the narrow membranous inter- 
space which exists between the broad pedate bases of the haemal arches and projects 
downwards into the cavity of the aortic canal (fig. 2). Anteriorly, this fibrous band 
blends with the cartilage of the basis cranii, but posteriorly it may be traced nearly to 
the end of the tail. 
As to the significance of this curious structure I can offer no suggestion, but it is 
not a little remarkable that it exactly corresponds in position and relations to the 
“sub-notochordal rod” which was first observed by Gotte in the embryo Frog, and 
subsequently and independently by Balfour and Semper in the developing Scyllium. 
In the Shark the “ sub-notochordal rod ” appears to be formed from a ridge of cells 
which runs along the medio-dorsal line of the alimentary canal. t The cells at the 
extreme summit of this ridge become gradually constricted off to form a linear rod, 
the splitting off taking place from before backwards. The formation of this rod by 
segmentation from the summit of the hypoblast is an almost exact repetition of the 
process by which the notochord itself is derived from the same germinal layer. When 
the dorsal aorta is formed, the “sub-notochordal rod” becomes separated from the 
summit of the alimentary canal, and the aorta interposed between the two. At the 
period of its fullest development the rod extends anteriorly to a point in front of 
auditory capsules, but terminates a little behind the apex of the notochord ; pos- 
teriorly, it extends nearly to the end of the tail. When at its maximum size the 
“ sub-notochordal rod ” lies immediately beneath the notochordal sheath and dorsad 
of the aorta ; and it even projects into the cavity of the latter, which consequently 
appears in transverse section as reniform in outline, with the hilum directed upwards. 
♦ 
* Jenaische Zeit. 1867. 
f Balfour, ‘Journal of Anatomy and Physiology,’ vol. xi. 
