142 
ME. W. K. PAEKER ON THE STRUCTURE AND 
riorly , where the nasofrontal process has been cut through a little to the right of the 
mid line, so as to expose its cavity, which contains gelatinous tissue surrounding an 
upper and a lower cartilage; these cartilages underprop the 1st cerebral vesicle (Cl). 
The form of the cheek, as seen from the outside at this stage, is simply swollen ; but it 
becomes ribbed soon afterwards (fig. 2, side view) ; this ribbing, however, is largely deve- 
loped already on the inner side (fig. 4) ; the subvertical thickenings contain the solidifying 
“visceral arches,” and th e furrows are the commencing visceral clefts: at present this 
dehiscence of the facial wall has not affected the cutaneous investment ; but the mucous 
membrane has been thrown into sharp folds, as is well shown in transverse sections 
(figs. 5-7). The side view of a recently hatched embryo shows in its further advance* 
ment much that is instructive (fig. 2) : in this figure the skin has been removed from 
over the auditory sac ( au .). 
Here it is seen that the three sense-capsules occupy what may be called the “ lateral 
cranio-facial line ; ” they follow the curve of the cranial cavity, and lie immediately 
above the visceral arches, the first of which is beneath the nose but above and in front 
of the mouth ; the postoral arches reach to the yelk-mass ( y .) which fills the thoracico- 
abdominal cavity ; and here the skin is already cleft on the outside ; this early dehis- 
cence is the first appearance of the opening between the last branchial arch and the 
anterior margin of the thoracic wall. The outer wall of the face is now rather flatter; 
and in lines corresponding with the internal ribbings (fig. 4) shallow sulci are seen, over 
which the skin is becoming attenuated. On about the middle of the 3rd and 4th post- 
oral arches there is a small bud ; this is the first appearance of the branchial papillae 
on the 1st and 2nd branchial arches, which are thus seen to be developments of the 
cutaneous system. If the skin be now peeled away from the rest of the cheek (fig. 3), 
we shall see what caused the ribbings in the moutli-cavity (fig. 4) ; here are displayed a 
series of parallel subvertical rods of rapidly hardening cartilage, which occupy the whole 
space from the middle of the fronto-nasal process to the thoracic yelk-mass. Here we 
see that the swollen condition of the cheeks seen in the front view (fig. 1) is not so much 
due to the size of the oral “ chasm,” as to the very solid walls of the face ; nearly all the 
tissue intervening between the oral lining and the outer skin has been expended in the 
formation of these bars, which are very closely packed together externally. It will be 
seen, both from the inner and outer view (figs. 4 & 3), that the shape of these bars is 
gently sigmoid, that they bulge outwards in their middle part, that they turn inwards 
both above and below, and that the foremost and the last two or three do not reach so 
far downwards as those in the middle. This arises from the fact that the foremost pair 
lie above and in front of the mouth, and the hinder arches lie above the pericardium. 
They thus follow the shape of the “ stomo-pharyngeal cavity,” of which they are 
the skeleton ; and they are by no means to be confounded with the arches that grow 
from the segmented vertebral axis, which, when finished afterwards, enclose the heart 
and all the other viscera. The point, first of all in importance, which is to be noted 
here is that these rods are all absolutely distinct (fig. 3) ; their upper termination 
