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ME. W. K. PAEKEE ON THE STEUCTUEE AND 
for the cutaneous system, now complete over both embryo and yelk-sac, is just ready to 
undergo very remarkable changes, becoming highly modified over the sense-organs, and 
undergoing dehiscence in the facial walls. 
At present, in embryos two lines long, still in the jelly-ball, the cutaneous system has 
only one opening, namely, the oral (Plate III. fig. 1, m.). 
Elevations and hollows there are, however, in abundance, which will show their own 
meaning afterwards, when dehiscence takes place, or even now, when they indicate the 
form of already developed organs or parts within. A front view of the skull and face 
of the unhatched frog-embryo (two lines long) is very instructive (see Plate III. fig. 1, 
X 20). 
In the centre is the small lozenge-shaped oral opening (m.), above this the “ fronto- 
nasal process ” ( f.n .), ending below in a right and left horn ; this median, bilobate region 
forms the inner half of the boundary of each of the rudimentary olfactory sacs ( ol . ). 
Below the oral opening we have the right and left cheeks or facial walls, swollen in 
this early stage ; for the fore part of the “ yelk-chasm ” is permanent, and enlarges to 
form the pharyngo-stomal cavity, which is already lined by a layer of cells, derived, 
according to Reichekt, from the lower part of the “ cumulus germinativus ; ” this layer 
becomes the mucous membrane of this region, and passes, some way within the oral 
opening, into the cutaneous system. The two sides of the face are separated anteriorly 
by a deep fossa ; below they terminate in those thick reduplications of the cutaneous 
system which are called the “ claspers” (cp.). 
On each side of the fronto-nasal region ( f.n .) there is a reniform depression covered 
by a thinner tract of the cutaneous system, and having as its inner rim the thickened 
edge of the fronto-nasal process ; above and on the outside the rim is still more strongly 
marked ; this is the rudiment of the olfactory sac (ol.). 
At present this is merely a mass of cells formed by modification (differentiation) of 
this part of the cutaneous system ; it is at present blind within and without ; but the 
depression will become a tube, and the rim will afterwards enclose the alinasal cartilage. 
Above, behind, and external to these rudiments there is another pair, having a like 
relation to the cutaneous system ; these become the eyeballs. 
The thick rim of these somewhat triangular depressions is a fold of the skin which is 
open in front, the structure of the eyeball being mostly horseshoe-shaped at first and 
circular afterwards. 
Surmounting the whole we have the frontal part of the cranial region, which contains 
at this part the “ 2nd cerebral vesicle ” above, and the “ 1st cerebral vesicle ” below, the 
“ mesocephalic flexure ” having turned the latter downwards and given the 2nd vesicle 
(fig. 4) a frontal position. 
This front view (fig. 1) shows a very sharp distinction between the frontal and nasal 
regions ; for the depression on the eye-rudiment is continued across the top of the face, 
the two seams meeting in an arched manner at the mid line. 
Towards this seam or “ raphe ” the cutaneous system is thickened both above and 
