62 FORMATION OF THE YOLK-SACK. 



The first cleft is no longer similar to the rest, but by the 

 closure of the lower part has commenced to be metamorphosed 

 into the spiracle. 



Accompanying the change in position of the first cleft, the 

 mandibular arch has begun to bend round so as to enclose the 

 front as well as the sides of the mouth. By this change in the 

 mandibular arch the mouth becomes narrowed in an antero- 

 posterior direction. 



In fig. H are seen the long filiform external gills which now 

 project out from all the visceral clefts, including the spiracle. 

 They are attached to the front wall of the spiracle, to both walls 

 of the next four clefts, and to the front wall of the last cleft. 

 They have very possibly become specially developed to facilitate 

 respiration within the egg ; and they disappear before the close 

 of larval life. 



When the young of Scyllium and other Sharks are hatched 

 they have all the external characters of the adult. In Raja and 

 Torpedo the early stages, up to the acquirement of a shark-like 

 form, are similar to those in the Selachoidei, but during the 

 later embryonic stages the body gradually flattens out, and 

 assumes the adult form, which is thus clearly shewn to be a 

 secondary acquirement. 



An embryonic gill cleft behind the last present in the adult 

 is found (Wyman, No. 54) in the embryo of Raja batis. 



The unpaired fins are developed in Elasmobranchs as a fold 

 of skin on the dorsal side, which is continued round the end of 

 the tail along the ventral side to the anus. Local developments 

 of this give rise to the dorsal and anal fins. The caudal fin is at 

 first symmetrical, but a special lower lobe grows out and gives 

 to it a heterocercal character. 



Enclosure of the yolk-sack and its relation to the embryo. 



The blastoderm at the stage represented in fig. 28 A and B 

 forms a small and nearly circular patch on the surface of the 

 yolk, composed of epiblast and lower layer cells. While the 

 body of the embryo is gradually being moulded this patch 

 grows till it envelopes the yolk ; the growth is not uniform, but 



