DEVELOPMENT OE THE SKULL IN THE SALMON. 
135 
(Plate IV. fig. 7). Upon the two “ hypohyals” the lingual element or basihyal Q/./j.) 
rests in its hinder part; its fore part projects into the substance of the tongue. 
The branchial arches (Plate IV. fig. 1, br. 1-5) are still unossified, but their basal 
elements are being fused together; the relation of the foremost piece to the floor of the 
mouth is shown in section (Plate III. fig. 10, b.br. 1), and the second pharyngo-branchial 
in its relation to the basis cranii is shown in another section (Plate V. fig. 5 , pi.br. 2) : in 
this section part of the first cerato-branchial region is cut through (c.br. 1), and here 
also is shown the articulation of the “ hyo-mandibular” beneath the ampulla of the 
“horizontal canal” ( h.m ., h.s.c.). 
Sixth Stage. — Young Salmon of the sixth week after hatching. 
In Salmon of this stage there has been an increase in length less than might be sup- 
posed from a month’s growth ; they are three or four lines longer than in the fifth stage 
(about an inch and a sixth), but the yelk-sac has been entirely taken into the abdomen, 
and the tissues have become very much more perfect and solid. Ossification has begun 
in the parosteal tracts generally, and in most of the ectosteal, but we shall not find the 
bony plates of the sphenoidal region for some weeks to come ; and most of the' ectosteal 
plates will be best described in Salmon of the first summer. Yet there are changes of 
great importance that have already taken place in the cartilaginous skull, which is now 
beginning to pass from the Polgpterine into the Salmonine morphological type. The 
vertical section (Plate V. fig. 2) is shown with the brain removed, and this must serve for 
comparison with both the inner and outer views of the fifth stage (Plate IV. figs. I & 4). 
The middle ethmoidal region is now much more developed and is very solid, the nasal sacs 
lying, as in the Sturgeon, in little recesses or crypts on either side. The trabecular cornua 
are only separate at their ends by a very small emargination; these, and the septum common 
to both the nasal and anterior orbital region, are formed of continuous cartilage. This 
section, which is a little more than half, being made to] the left of the exact mid line, 
shows what is most instructive, namely a double origin for the interorbital cartilage. The 
“ mesoethmoid,” besides being continuous with the prosencephalic cartilaginous roof, 
also sends a sharp, wedge-like lamina backwards into the presphenoidal region ; and 
besides this the coalesced tilted trabeculse have sent upwards another lamina, which 
runs from the ethmoidal to the basisplienoidal region, some distance below the pre- 
sphenoid. Here, evidently, we are beginning to get a clue to the remarkable characters 
of the Bird’s skull, in which the differentiation of morphological regions often takes place 
some days after the formation of continuous hyaline cartilage over several truly distinct 
parts. In the Bird ( chick ) the distinctness of the trabeculce from the “ investing mass” 
is best shown during the second week of incubation, and the morphology of the inter- 
orbital plate is best studied at the beginning of the third. I have shown this breaking 
up of large parts of the cartilage into more or less distinct morphological elements in 
the “shoulder-girdle” in my memoir on that part of the Vertebrate skeleton; and it 
must not be supposed incredible that morphological differentiation of the skull may 
MDCCCLXXIII. T 
