OF THE SKULL IN SHAEKS AKD SKATES. 213 



On the Steuctuke aivd Development op the Skate's Skull. 



Fii'st Stage : Enihryo of Raia maculata, \\ inch in entire length, seven weeks after 

 deposit of egg; and Ernhryos q/" Pristiurus, f and f 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 (PI. 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 (PI. XXXV. fig. 1, and PI. XXXIX. fig. 5) are simple lunate folds 

 on each side of the umbilicus (m) ; 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 emhnjological processes have been gone 

 through ; but now its primordial skeletal parts have been fairly difl'erentiated. The 

 sacs of the special sense-organs are at present horseshoe-shaped folds of the embryonic 

 cuticle and cutis, the large closed brain-vesicles (6'i, 6'2, C'3)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 

 complete. 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 XXXIX. figs. 1, 2, 5, hr). 



Behind the azygous oral cleft the visceral openings are variable in size, the first post- 

 oral cleft (the " spiracle," or " tympano-Eustachian," cl 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 {cl 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. i.— March 1st, 1878. 2 h 



