DEVELOPMENT OF THE SKULL IN THE SALMON. 
119 
principal strata of the blastoderm are seen above this space, and between them another 
shallow transverse vacuity, the mouth. 
The anterior cerebral vesicle (c. i) is full of a gelatinous fluid, but its more condensed 
peripheral part is very soft and of no great extent ; it is somewhat grooved below. Below 
this shallow sulcus, in the tissue between the membranous cranium and palatal skin, 
there are seen the sections of two chondrifying bands, and in the sulcus between the eye 
and brain a thickening of a triangular form is observable ; this is softer still. The inner 
pair are the trabeculae in section, and the outer the subocular or palato-pterygoid bars. 
In the thin “ hammock,” with its thickened middle part, which is swung at a small 
distance below the space of the mouth-cavity, there is a more solid but flat pair of 
sections ; these are the outspread ends (see also fig. 7, mn .) of the first postoral rods, 
their symphysial ends. So that we have here, beneath the first cerebral vesicle : — 1st, 
a capsule ; 2nd, a layer of “ mesoblast,” containing the cartilaginous rudiments of the 
first and second prseorals ; 3rd, a layer of enderon (palatal) ; 4th, below the oral cavity 
two layers of epiblast and one of mesoblast, this latter containing the first postorals or 
third facial arches. A section from the same part in a more advanced embryo, or 
near to the second stage, shows a great change (Plate II. fig. 1) ; here the section, 
which is through the very middle of the exposed and hidden eyeballs, is also posterior 
to the “ prosencephalon ” and through the middle of the “ thalamencephalon their 
imprisonment has strangely hindered their symmetrical growth, but they have acquired 
much substance ; we are here behind the cavity of the prosencephalon, and in front of 
the next, and therefore in the most solid part of the brain. The left half of the “ thalam- 
encephalon,” like the left eye and the left half of the “hemispheres,” is smaller than 
the right. The more solid tissue of the first, second, and third arches is shown in section, 
and the flattened Meckelian region is well displayed at the symphysis of the chin. A 
glance at this figure and the last, for comparison’s sake, will show how fast the dif- 
ferentiation of layers has taken place ; but that which is of most importance to notice is 
that a new cavity has been formed, the “ anterior palatal recess ” (a/jj.r.). This marked 
splitting-up of the facial strata divides the mouth from the layer in which the trabeculae 
and subocular rods are imbedded, and is well seen in older embryos as a recess between 
the upper (trabecular) palate and the “ naso-frontal process ” (see Fourth Stage, Plate II. 
fig. 10, n.f.p ., a.jo.r.). 
The next section (Plate I. fig. 11) is through the fore part of the right ear-sac, but 
the obliquity of the head makes even this true transverse section miss the left ; it is in 
front of it, and also of the notochord. In this stage the eye- and ear-balls overlap each 
other, and hence a section which barely escapes the eye takes off the fore part of the 
ear. The brain is here somewhat more developed than in figs. 8 & 10; but the razor 
passed through the more solid cerebral substance which lies immediately behind the 
thalamencephalon, in front of the pituitary body, and across the overhanging fore edge of 
the “ mesencephalon.” The razor missed the “ subocular ” boss on both sides, whilst the 
trabecuke, the mandibular bars, and the right hyoid were cut through obliquely. On the 
mdccclxxiii. E 
