THE ELASMOBRANCH FISHES 



11 



At an early stage there appears above the germinal disc a horseshoe-shaped 

 mass of tissue, the closed end of which represents the head end, and the open 

 end, the tail end. This mass of tissue then becomes spatulate (see p. 230, 

 fig. 209). In a further stage, in which the body takes on definitive form, the 

 two types are characteristically similar. In both, the optic vesicles stand out 

 as prominent structures and the gill clefts, in breaking through, occupy about 

 the same lateral position. 



We may figure two stages (fig. 22a-d) which represent the parting of the 

 ways. The first (a) and third (c) of these are of Acanthias; the second (b) 

 and fourth (d) are of Urolophns. While in Urolophus (fig. 22b) the clefts still 



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ABC D 



Fig. 22. Development of body form in Acanthias and Urolophus. 



A. Stage in development of SquaJus acanthias. (Length 20.6 mm.) (From Scammon.) 



B. Stage in development of Urolophus halleri. (Length 22 mm.) (C. G. Potter, del.) 



C. Older stage in development of Squalus acanthias. (Length 28 mm.) (From Scammon.) 



D. Older stage in development of Urolophus halleri. (Length 38 mm.) (C. G. Potter, del.) 



open on the sides, other changes are taking place which immediately charac- 

 terize it as a ray. The most notable of these changes is the extension of the 

 pectoral fin. 



In the figures of Acanthias (c) and of Urolophus (d) both assume essen- 

 tially the features of the adult. While Acanthias retains its slender form, 

 Urolophus becomes greatly flattened, the disc-shape being the result largely 

 of the growth of the pectoral fins. Each pectoral now takes the shape of a 

 battle-axe, the blade of which extends outward. The posterior point of the 

 blade projects toward the pelvic fin, while the anterior point extends over the 

 branchial region. As the fin broadens, its anterior tip meets the growing antor- 



