in the Fcetus of Vertebrated Animals. 97 



part of the arch which joins the first pair of branchial veins at 

 the place where the root of the aorta begins. 



The anterior branchial plate, or that situated between the 

 mouth and the foremost branchial apertures (Figs. 2, 3, a) 

 instead of being covered by the fringe of leaflets proper to 

 the gills, projects farther backwards in the neck, and begins to 

 overlap the branchial plates posterior to it (Fig. 4, 5 a). Ac- 

 cording to Rathke, the anterior part of this plate forms the 

 lower jaw ; the posterior part, continuing to extend itself back- 

 wards, forms the opercular covering of the gills. 



The branchiaa of the Rays and Sharks, again, consist of five 

 pairs of double branchial plates, from the edges of which rows of 

 leaflets are suspended ; but, so far as I know, the mode in which 

 they become developed has not been observed. It would be in- 

 teresting to know in what manner the opercular fold and open- 

 ings of the gills are formed in these animals, as the smallness of 

 the lower jaw and the absence of the lateral processes of the 

 lingual bone, seem to indicate that only five branchial plates 

 originally exist in the foetus, and that all of them become de- 

 veloped in the formation of the branchiae of the adult animal. 



The foetus of cartilaginous fishes besides possessing a yolk-sac, 

 in the vessels of which a large quantity of blood is spread out, are 

 provided, during a considerable period *of their foetal life, with 

 other organs, by means of which the aeration of their blood is 

 promoted. The External Gills or branchial appendages were 

 known to the late Dr Monro *, and are described as such by 

 him in the skate, (see Fig. 6, A. H). The connection of these 

 appendages with the hoops of the internal gills, was, however, 

 first shewn by Dr Macartney of Dublin in the foetus of the 

 shark. They consist, according to this anatomist, of five bundles 

 of tender filaments hanging from each side of the neck. These 

 bundles appear to arise from the external integuments, but are 

 really attached to the internal gills on the inner side of the 

 branchial apertures -f. 



In some beautiful specimens of the external gills of the Squa- 

 lus Catulus and S. maximus, which I had lately an opportu- 

 nity of seeing in the Museum of the College of Surgeons, Lon- 



• Monro on Fishes, plate XIV. f Journal de Physique, Fevrier 1818. 

 OCTOBER— DECEMBER 1830. G 



