126 
ME. W. K. PARKER ON THE STRUCTURE AND 
curved beam on which the first vesicle rests. The trabeculse end in and "fill the cavity 
of the “ naso-frontal process,” which ends below and behind in a free lip — the floor of the 
“anterior palatal recess” (see also Plate II. fig. 10, a.jp.r.). In the section of the more 
advanced specimen the “subocular” (pterygo-palatine) rod is brought into view; 
between it and the trabeculae there is seen the bulging orbital floor (see also Plate II. 
fig. 10, and Plate III. fig. 2, or.). Here, under the eyeball (see also Plate III. fig. 4, 
l.clfl is a cleft , which 1 find very distinct, and which Professor Huxley shows me is the 
“lacrymal cleft;” it is the true first cleft ; the oral opening is the second, and therefore 
the tympano-Eustachian, or first postoral, is the third. This cleft is also important as 
stamping the true visceral-arch character upon the two prseorals. When arches can be 
shown me which combine the characters of ribs and visceral arches , then I will treat of 
them as morphological varieties ; species they are, to all intents and purposes, being 
altogether independent of the perineural axis — most largely developed when furthest in 
front of it, and excluding the heart instead of containing it. The trabeculae at the time 
of hatching are solid, so also are the first and second postorals ; but the chondrification 
of the pterygo-palatine and branchial arches is tardy, and they still contain a central 
cavity filled only with protoplasm (see Plate II. fig. 10, ppg., and Plate III. fig. 3, hr.). 
Morphologically, we are now at the level of the “ Chondrosteous Ganoids;” a familiar 
example, happily extant, is the Sturgeon. Now in the life-history of this Teleostean 
we for the first time see the “ stylo-hyal,” and the Sturgeon, standing above the Plagi- 
ostomes and below the higher Ganoids, shows it for the first time, zoologically. Even 
in the second stage the Salmon has gone beyond the Plagiostom.es in having segmented 
off a “ hypohyal ; in this it agrees with the Sturgeon, which, however, has its own speci- 
alization in the complete subdivision of the first half of the second postoral, the “ hyo-man- 
dibular.” Even at the time of hatching, the first postoral cleft is being obliterated, but 
the cutis being removed, it can be seen as an opening undergoing division into two : 
the lower of these is the tympano-Eustachian, and is entirely occluded by the sym- 
plectic ; the upper part remains open in most Plagiostomes and Ganoids as the “ spiracle.” 
The amount of metamorphosis which takes place before the fry has fairly escaped from 
the egg is evident (see Plate III. figs. 1 & 2); for almost all the essential characters 
of the Teleostean face have already appeared. The most remarkable of these are the 
low position of the first postoral ( mtpg ., q.), the equally low position of the posterior 
division of the second postoral ( c.h .), the appearance of the little connecting segment 
( st . h.), and the development of the halved hook of this, the hyoid arch, into the back- 
wardly turned massive head of the “ hyo-mandibular” ( h.m .). Moreover the individual 
segments have not only gained their new relationships, they have also acquired very nearly 
their permanent form ; for the muscular masses are now developed, which have their 
origins and insertions in harmony with the nature of this type of Fish. 
The huge eyeballs still obscure the morphology of the upper part of the face, and 
their sockets especially, at present over large (see Plate II. fig. 10, and Plate III. 
fig. 2, or.), widely sever the pterygo-palatines and their cleft from the trabeculae. 
