458 
MR. W. K. PARKER ON THE DEVELOPMENT 
Fourth Stage ,— Young Lepidosteus, 11 \ to 12^ lines long: average size 1 inch. 
In these, the largest of those reared by Professor Agassiz and Mr. G arm an, the 
chondrocranium is perfect; the occipital arch is beginning to ossify, and the investing 
bones are very numerous and quite distinct. 
The cranium at this stage (Plate 32) corresponds very closely with that of a young 
Sturgeon Jive inches long, but has much larger membranous tracts, and is altogether a 
much lighter structure ; in having rudimentary basi- and ex-occipital bony centres, it 
has already gone beyond the skull of an adult Sturgeon. 
The fissure between the fore and hind brain (Plate 32, fig. 4, C 1 ., C 3 .) is very 
distinct, and reaches to the base of the mid brain (C 2 .); but there is no “ posterior 
clinoid wall, ’ such as would exist and be very massive in the skull of an embryo 
Sauropsidan or Mammal at the same stage. 
Already the notochord (Plate 32, fig. 4, ne.) has retreated to a considerable distance 
behind the pituitary body (py.), which is now an appendage to the infundibulum ( inf. ); 
the brain well fills the whole cranial cavity up to this stage, but the hemispheres 
(CA) are relatively very small. 
Another thing to be noticed is this, namely, that the pre-cerebral growth of cartilage 
is almost as long as the whole cranial cavity, although it is only a fraction of the length 
to which it will attain. At first sight it might be thought that the mesocephalic 
flexure was gone, but the up-throwing of the mid brain, and the meeting of the fore 
and hind brain, show that the bend is very large and very sharp at one point. The 
four faces of the skull are all largely membranous, and but for the notochord (nc.), the 
floor would be open along nearly its whole length, for the cartilage only closes in at 
the mid line beneath the front end of the hemispheres and the olfactory lobes (CA, 
CA). The thick cranial notochord (nc.) is receiving a bony investment between the 
thin, post-auditory ends of the investing mass (iv.) ; this will be seen better in the 
transverse sections (Plate 33). The fore ends of the basal bars (figs. 2, 3, iv.) diverge 
from the notochord (nc. ) some distance behind its apex; in the middle part they are 
completely confluent with the auditory sacs. 
The narrowed, diverging bars that retreat from, and then shoot on far in front of the 
notochord, are the trabeculae (tr.); they approximate gently, and their interspace in 
front is sharply pointed. But the trabeculae have not merely approximated, they are 
united together by the intertrabecular wedge (see Plate 31, figs. 7, 8, i.tr.) ; and this 
has now become a large rod, running forwards to the end of the narrow snout. 
Outside this thick rod, but little of which is formed by the lateral bars, those bars 
grow externally into a large lanceolate leaf of cartilage, which reaches right and left 
nearly to the small, distant nasal sacs (fig. 5, ol.). These peculiarly Acipenserine out¬ 
growths of the trabeculae are the familiar “ cornua ” ( c.tr .) curiously modified ; both ends 
of each leafy growth are free, as rounded ears of cartilage; on the inside there is a sulcus, 
deepest, on both surfaces, between each cornu and the coalesced bars in the middle. 
