DEVELOPMENT OF THE SKULL OF THE COMMON FROG. 
141 
below, so that where the skin of the forehead meets that of the top of the face there is a 
very evident selvedge ; whilst the furrow between the “ raphe ” is of a paler colour, and is 
but little differentiated from the gelatinous blastema which separates the membranous 
cranium from the oral mucous membrane*. 
The lips, which are so highly developed afterwards, are at present merely represented 
by the somewhat inturned edges of the oral opening ( m .), the upper being formed by 
the emarginate lower edge of the fronto-nasal process, and the lower on each side by the 
bevelled supero-internal angle of the descending facial plate. These facial plates are 
greatly inturned at the mid line, where they form an obliquely descending raphe which 
is but imperfectly finished above ; the oral opening being continuous with a slit which 
descends for some distance, the right and left facial walls being imperfectly soldered 
together above. 
If a vertical section of the head of an embryo at this stage be made, we have the 
appearance seen in fig. 4. 
The thick dermal layer (d.) is seen to follow the inflections of the membranous cranium 
( m.c .), which has already a considerable consistence, and the inflections of which relate 
to the form of the enclosed cerebral vesicles (fig. 4, C 1, C 2, C 3) and the rudimentary 
pituitary body (py.). The flexure of the brain upon itself (mesocephalic flexure) is 
shown in this figure, although its straightness is in some degree recovered. Underlying 
the medulla oblongata ( m.ob .) and medulla spinalis ( m.s .) there is a thick rod of gela- 
tinous tissue enclosed in its own sheath ; it is blunt-pointed and decurved, and it termi- 
nates a little distance behind the pituitary vesicle (py.) ; this is the notochord (n.c.). 
The tissue forming this rod is very similar to that which everywhere fills up the spaces 
between the rudimentary organs in the embryo at this stage ; it is a very watery kind of 
blastema, interspersed in all directions with delicate membranous bands, a structure well 
displayed in Mullee’s figure of a transverse section of the notochord of the Hag-fish 
(. Myxine glutinosa ) (see ‘ Myxinoids,’ pi. 9, fig. 1). 
Already, patches of cartilage have appeared in the outer part of the notochordal tube 
(sheath) ; the foremost of these become the postpituitary part of the basis cranii, and 
the following patches form vertebral arches. In this section the yelk-mass (y.) is seen 
to persist up to nearly the fore end of the notochord above, but lower down it is deficient 
further back ; this is caused by the bulging behind of the mouth-chasm or stomo-pha- 
ryngeal cavity. 
Still further down the substance of the yelk itself has undergone transformation, 
being applied to the formation of the rudimentary heart and its sac (better seen in next 
stage, fig. 12 , p.c.d.). Beneath the sac the cutaneous system has joined from the right 
and left side and has produced the thick claspers ( cp .). 
In this section the oral mucous membrane is shown lining the cheek, except ante- 
* Professor Huxley, at first sight of this figure, pointed out to me that the middle part of this “ raphe ” 
occupies the place of the azygous nasal opening of the young Lamprey (see Muller’s £ Myxinoids,’ pi. 4, figs, 
9, 10,/). 
