ELASMOBRAXCHII. 51 



Accompanying the change in position of the first cleft, the man- 

 dibular arch has begun to bend round so a;s 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 pro- 

 ject 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 is less rapid in tlie immediate neigh- 

 bourhood of the embryonic part of the blastoderm than elsewhere. 

 As a consequence of this, that part of the edge, to which the embryo 

 is attached, forms a bay in the otherwise regular outline of the edge 

 of the blastoderm, and by the time that about two-thirds of the 

 yolk is enclosed this bay is very conspicuous. Jt is shewn in fig. 

 30 A, where hi points to the blastoderm, and yk to the part of the 

 yolk not yet covered by the blastoderm. The embryo at this time 

 is only connected with the yolk-sack by a narrow umbilical cord ; 

 but, as shewn in the figure, is still attached to the edge of the 

 blastoderm. 



Shortly subsequent to this the bay in the blastoderm, at the 

 head of which the embryo is attached, becomes obliterated by its 

 two sides coming together and coalescing. The embryo then ceases 

 to be attached at the edge of the blastoderm. But a linear streak 

 formed by the coalesced edges of the blastoderm is left connecting the 

 embryo with the edge of the blastoderm. This streak is probably 



4—2 



