DEVELOPMENT OE THE SKULL IN THE SALMON. 
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latest and best abstract of what is known as yet of the development of the Vertebrata ; 
this is given in Professor Huxley’s new work entitled “ A Manual of the Anatomy of 
the Vertebrated Animals” (see pp. 3-13). 
In the younger embryos which formed the subjects for my first stage , the “ primitive 
groove ” had not closed over the cephalic (Plate I. figs. 2, 4, 5) nor over the caudal 
extremity of the long tape-like germ. 
At present the rudiments of the nose, eyes, and ears are very imperfect, the olfactory 
sacs are merely (Plate I. fig. 1, ol.) pits surrounded by a circular ridge of the “ epiblast,” 
the folds of the eyeball (e.) are not coalesced, and the auditory involutions (figs. 1, 3, 
6, 9, au.) are still widely open. 
The peripheral portions of the “ blastoderm ” extending over the yelk form a bag so 
wide open above that its rim reaches to within a short distance of the posterior margin 
of the mouth, close behind the converging Meckel’s cartilages (Plate I. fig. 1, u,v., mn.). 
In this early stage all the principal parts of the head have their rudiments differentiated ; 
but these rudiments lie in the midst of a very soft stroma, and much care has to be 
taken in hardening and even colouring the specimens before the various organs can be 
made out. The most immature embryos worked out by me are illustrated by figs. 1 & 2, 
Plate I. ; and although the parts which have begun to form the skull and face are 
coloured as though they were cartilaginous, yet their actual condition was merely that of 
more consistent tissue than that in which they were imbedded. This tissue was indeed 
composed of the “ mother-cells,” very small, of hyaline cartilage ; and it could easily be 
seen, in the case of the facial arches, that the apparent rods were merely tubes composed 
of finely granular matter, lying in and also enclosing a thoroughly diffluent tissue. 
As to degree of development, the first two pairs behind the mouth were the rods most 
distinct, next to them the branchial-arch rudiments, then the trabeculse, and lastly the 
“ subocular ” bands, the rudiments of the pterygo-palatine arch. 
My illustrations will appear to the reader as representing strangely twisted, oblique 
objects; they are true to Nature, however, in that the upper and lower planes of the 
head, in the unhatched Salmon, are placed in such an oblique manner that only one 
eye-dot appears when the eggs are examined with a pocket lens (see Plate I. fig. 4, the 
head seen from above). Hence all bird’s-eye views of the head, whether upper (fig. 2) 
or lower (fig. 1), are strangely unsymmetrical ; and it is useless at present to seek in these 
imprisoned young for either the swelling cerebral vesicles, with which the embryologist 
is so familiar, or for that very important tilting-over of the fore part of the brain, causing 
the “ mesocephalic flexure.” 
The flat edge of the down-kept membranous cranium is seen from below like a second 
superciliary ridge (Plate I. fig. 1). One eyeball, it may be indifferently either right or 
left, is fully seen below, whilst the other is mainly seen above, and only peeps down to 
the lower plane. So large are the ear- and eye-sacs that they overlap each other ; this 
is seen in specimens that have suffered no compression. Most of the structures with 
which I have to deal are formed in what I would call secondary strata of the “ 
Q2 
meso- 
