DEVELOPEMENT OE FISHES. 



609 



TTcll as the internal 



425 



This, as a rule, is the form antl structure acquired by the tail in 

 existing Teleostomous Fishes: but the 'heterocercal ' modification 

 does not intervene between the proto- and homo-cereal ones in 

 the GadidcR. 



The pectoral fins are developed usually before extrication, and 

 are often of large relative size : in this respect, as well as in the 

 inferior position of the mouth, in the unsymmetrical form of the 

 tail, in the gristly skeleton, and uncovered gill-slits, the embryo 

 Salmon, Pike, Perch, &c., manifest transitory characters which 

 are permanent in Sharks. 



The singular productions of the rostrum in most Plagiostomes, 

 like the elongation of the jaws in osseous species, are later 

 phenomena of developement. It is interesting to find the broad, 

 depressed, obtuse embryonic form of head common to many of the 

 Fishes of the old red-sandstone. M. Agassiz thus accounts for the 

 extreme rarity of the Ichthyolites of this formation presenting a 

 profile view of the head : it lies in most cases upon the upper or 

 the under surface of the body. 



All the Plagiostomes have the external as 

 division of the vitcllicle, fig. 425 ; 

 the peduncle of the external one 

 d, is longer, in some species con- 

 siderably so, than in Osseous 

 Fishes, and it is beset with villi 

 in Carcliarias and Zygana} 

 The tegumcntary covering of the 

 outer yolk, ib. d! , is denser and 

 more ojiake in Plagiostomes : 

 the inner yolk, ib. e, is co- 

 vered only by the proper vitel- 

 line tunic, which is thin and 

 transparent : it communicates 

 with the short tract of small 

 intestine which intervenes be- 

 tween the pylorus and the val- 

 vular straight gut, li : it receives 

 the external yolk, d', as this is 

 ijroo-ressively squeezed into the abdomen by the contraction 

 and interstitial absorj)tion of its tunics, c' : and, as no part of the 

 fo3tal abdominal appendage is cast off, nor the chord divided, there 

 is no cicatrix — no umbilicus. The arterial vessels of the yolk are 

 derived, not from the mesenteric vein as in Osseous Fishes, but 



' cxsiii. tf. iii. 



VOL. I. E R 



Eiiibvyo Cartilaginous flsli, Si-'ijUUnn. 



