DEVELOPMENT OF THE SKULL IN THE SALMON. 
123 
used as a rule. Yet this lesson was learned from the Frog — namely, that part of an arch 
being segmented off from the primary bar may travel up or travel down just where the 
specializing force pleases ; the “ hyoid cornu ” of the Frog travels up, that of the Osseous 
Fish travels down. The anterior division of the second postoral in the Salmon now 
stands forwards and touches the first postoral arch (Inn., q.) ; it has left its own distal 
segment, the “ hypohyal ” ( h.hy .), to the hinder moiety, which moiety is still articulated 
like the fore piece, by a flattened hook, to the ear-capsule ; the “ basihyal,” or tongue piece 
( g.h .), still keeps its relation as the keystone, and still projects forwards considerably. 
But that which marks this stage is the segmented condition of the first “ postoral ” or 
mandibular arch (compare Plate II. fig. 3 with figs. 6 & 7). In this simple morphological 
process my former subject failed for interpretation ; there , in the Tadpole, the pier is of 
extreme length, so long as to carry the “ orbitar process ” in front of the eye ; but the 
free Meckelian arch is for some time very small indeed (op. cit. Plate in. fig. 12, and 
Plate iv. figs. 7 & 8, mk.) ; and although it acquires the sigmoid bend and the “ angular ” 
hook, yet at the first it is a mere hud pinched off, as it were, from the lower end of the 
bar, which before segmentation (op. cit. Plate in. fig. 3, and Plate iv. fig. 1,2, mn.) is 
strongly inturned below. Plere there is something very chirurgical in the way in which 
this segment is “ cut out without hands,” the invisible knife passing now across, now 
along, and then dexterously slanting through, so that a hall is formed on the upper part, 
and a cup and a long rounded angular process on the lower. This specimen is of the 
short-jawed variety (compare Plate I. figs. 1 & 3), and Meckel’s cartilages pass athwart 
and then curve backwards, perfectly Bay-like. Already the pier of the mandibular arch 
is escaping downwards from its original position, and has grown very little since the last 
stage, and, in this specimen at least, the orbitar process is less marked. If the position 
of this pair of cartilages be considered, it will be seen that they well illustrate the so-called 
palatal cartilages (“ Gaumenknorpel ”) of Narcine (Muller, op. cit. plate 5. figs. 3 & 4, e ) 
and the azygous palatal cartilage (“ unpaariger Gaumenknorpel ”) of the Sturgeon (op. 
cit. plate 9. figs. 10 & 11, h). It is the top part, the inturned outspread apex of the arch, 
which is segmented off in Narcine on each side ; in Acipenser it is evident that these 
segments early coalesced to form the “ azygous metapterygoid”*. It is perfectly normal 
that the facial arches should turn in and flatten out at their apex ; those the least spe- 
cialized, the gill-arches, constantly do this, and the substitution for gill-papillse of small 
teeth is a very gentle change and superaddition of function. Certainly the foremost 
arches are subject to most modification; but even the outflowing of the trabeculae is per- 
fectly normal, and their hooked apices only enclose the pituitary body because of its 
contiguity ; the hooking was a morphological fact before any such relation was brought 
about by specializing growth. As to the segmentation of the mandibular arch in Narcine 
and Acipenser, let the reader compare their metapterygoid segment, just referred to in 
Muller’s work, with his plate of the Chimcerds skull (op. cit. plate 5. fig. 2, qqq), and 
see how in that type the “ pliaryngo-branchials ” are segmented off to form outspread, 
smooth, arched roof-plates to the pharynx ; thus the order, harmony, and simplicity 
* See ‘ Monthly Microscopical Journal,’ June 1873, plate 20. figs. 1, 2, 5, mt.pg. 
