OF THE SKULL IN SHARKS AND SKATES. 213 
ON THE STRUCTURE AND DEVELOPMENT OF THE SKATE’S SKULL. 
First Stage: Embryo of Raia maculata, 13 inch in entire length, seven weeks after 
deposit of egg; and Embryos of Pristiurus, 3 and $ of an inch in length. 
The first embryo, taken for me from the egg-pouch of the Spotted Skate in the 
Brighton Aquarium by Henry Lee, Esq., was, in development, intermediate between 
the less and more mature embryos of the Dog-fish (from the same friend) already 
described (Pl. XXXIV.) ; the two others, the gift of Mr. Balfour, were from the Naples 
Aquarium. The length of the embryo Rays from Brighton was much greater than 
those of the Dog-fish, owing to the extreme development of the tail, the anterior part 
being no bulkier than the smallest embryos of the Dog-fish which I have just described. 
The pectoral fins (Pl. XXXV. fig. 1, and Pl. XXXIX. fig. 5) are simple lunate folds 
on each side of the umbilicus (w); and these embryos, if they had been found detached, 
could not easily have been distinguished from those of a Shark, the peaked rostrum 
and the fan-like shape of the fins not being developed as yet. 
At present the skeleton of the embryo is quite granular and transparent, so that by 
careful management most of its structure can be made out without any dissection. 
These embryos show, on a good scale, the structure of a vertebrate embryo in its 
first or simple morphological stage. Many embryological processes have been gone 
through ; but now its primordial skeletal parts have been fairly differentiated. ‘The 
sacs of the special sense-organs are at present horseshoe-shaped folds of the embryonic 
cuticle and cutis, the large closed brain-vesicles (C1, C?, C°) are full of watery fluid, and 
the third of these (C%) is covered very thinly by large soft mother cells. The mouth 
and pharynx are covered above by the axial structures, and floored below by a con- 
tinuous throat-skin, above which, behind, is the heart; but the sides are an open 
grating, hedged in by bowed bars. ‘The mesocephalic flexure is perfect, and the mouth 
ecmplete. None of the visceral arches meet, right and left ; but the pterygo-mandibular 
bars are coming near each other both in front of and behind the oval mouth. Behind 
the mouth the visceral bars are yet further and further apart, and the arches themselves 
’ gradually lessen in size; above, the hinder arches are set on to the infero-lateral edge 
of the vertebral structures in the cervical region; half the postoral arches are behind 
the third cerebral vesicle (Pl. XX XIX. figs. 1, 2, 5, dr). 
Behind the azygous oral cleft the visceral openings are variable in size, the first post- 
oral cleft (the “spiracle,” or “ tympano-Eustachian,” c/ 1) is less than the next; it has 
a pear-shaped outline, and soon fills up below, so that, when developed, this opening, 
the “spiracle,” is seen in the dorsal region, close behind the eye, whilst’ the others 
are on the ventral aspect. The second cleft (c/ 2) is larger, and retains its lower slit- 
like part. The remainder, between the proper branchial arches, are tolerably even in 
size, but have less vertical extent behind because of the shortening of the bars. From 
VoL. X.—ParT Iv. No. 4.—WMarch 1st, 1878. 24 
