OE THE SKULL IN THE URODELOUS AMPHIBIA. 
533 
This must suffice as an introduction to my first morphological stage ; but the papers 
just quoted should be studied before the details here to be given are gone into. 
My drawings only reach to the middle of the yolk-mass, a large quantity of which 
has not yet been used up. 
The head is bent down at a right angle by the “ mesocephalic flexure.” The hind 
brain (C 3 ) is long and lanceolate, its direction is straight. The mid brain (C 2 ) is 
large and rounded ; it is frontal in position, forming the actual end of the embryo. 
The fore brain (C 1 ) is egg-shaped ; it is half the size of the mid brain, and its direction 
is downwards, with its front margin considerably behind the front of the mid brain. 
The upper half of the huge hind brain is occupied by the vesicle of the fourth ventricle, 
and the cranial roof over this part is very thin, showing the cavity in the external views 
(figs. 1 & 2). These vesicles and their contents are epiblastic products ; between them 
and the epidermis (also epiblastic) there is a stratum of mesoblast which will become 
differentiated into “ cutis vera,” cranium, brain-membranes, and vessels. 
Rudiments of the chief organs of sense are now visible : the foremost pair are the 
nasal sacs ; they are now circular elevations of the skin, with a shallow, saucer-like 
depression in the middle ; each rimmed rudiment is placed on the hinder part of the 
lateral depression between the hind and fore brain. Correlated with the budding of 
the “ optic vesicle,” right and left, from the fore brain is an elevation of the outer 
structures ; a mass of cells in shape like a kidney, with its notch looking downwards 
and backwards, is placed in the lower part of the valley which runs between the hind 
and mid brain. This rough shetch as it were of the outer part of the eyeball shows 
that the lens is only partially developed. 
A very similar mass of cells, but rounder, is seen on the side of the hind brain ; at its 
middle this rounded mass has an imperfect rounded opening above ; this is the ear-sac 
(au.), and the opening is the primary involution or “ aqueduct.” 
From the outside, also, we can see the rudimentary, facio-branchial arches, with the 
commencements, between them, of the visceral clefts, and in front of them, below, that 
azygous, transverse dehiscence of the facial floor which becomes the “ deuterostomatous ” 
mouth. 
The mandibular and hyoid arches are marked out externally (figs. 1 & 3, mn., Jiy.) ; 
but the arches that follow — the branchials — are largely hidden by the opercular curtain 
which grows backwards from the hind edge of the hyoid arch (op.). 
In the side view (fig. 1) the upper part of the mandibular arch is seen as an arched 
thickening of the facial wall, below and behind the eye, and having a backward bend. 
In the recess between the thickenings for the eye and the nose there is a small elevation ; 
this is all that is seen at present of the “ maxillary rudiment.” In the lower view and 
in the section a band of skin is seen on the front margin of the oral region, behind the 
fore hrain; this is the “ naso-frontal process”; it becomes the upper lip, but acquires 
no labial cartilages; ultimately it will have growing into it the fore ends of the 
trabeculae in the middle, and laterally their out-turned cornua; these do not exist at 
present. 
