DEVELOPMENT OF FISHES. 301 



tunics : and, as no part of the foetal abdominal appendage is cast off, 

 nor the chord divided, there is no cicatrix — no umbilicus. Tlie 

 arterial vessels of the yolk are derived, not from the mesenteric vein 

 as in osseous fishes, but from ramifications of a brancli of the me- 

 senteric artery, and the blood is returned to the mesenteric vein. The 

 Ilunterian preparations of tlie embryo Carcharias (No. 1061), 

 Scyllium (No. 3250), Spiyiax (No. 3255), and ^/o7J^V/5 (No. 3261), 

 demonstrate another foetal peculiarity which later researches * have 

 shown to be probably common to all Plagiostomes, viz. the external 

 fringe of filaments developed from the branchial surfaces (i) : a tuft 

 extends out of each aperture, and even from the spiracula (o) in the 

 genera, Avitli those accessory openings. Each filament contains a 

 single capillary loop | : tliey disappear early, being removed by 

 absor2)tion. The last remnants may be seen in the foetal Saw-fish 

 {Pristis, No. 3263), which is eight inches in length, including the 

 saw, and has the duct of the external vitellicle attached. In the 

 oviparous Sharks, the branchial filaments re-act on the sti'eams of 

 water admitted into the egg by the apertures {Jig. 81. c). In the 

 ovo-vivipai'ous Sharks the size and position of the cloacal apertures 

 of the uteri would seem adapted to allow free ingress of sea-Avater (No.«- 

 3255) ; so that, whilst the vitellicle administers to the nutriment of 

 the embryo, the external branchiaj may perform the respiratory func- 

 tion. In the species of Shark, the smooth Emissole, in which Prof. 

 JVIiiller has shown that vascular cotyledons are developed from the 

 vitelline (omphalo-mesenteric) capillaries, wliich are firmly connected 

 to the uterine cotyledons, the vitellicle, like a true placenta, may per- 

 form both the nutrient and respiratory functions : the external branchiae 

 disappear some time before the exclusion of the Embryo and the 

 absorption of the yolk. In the Lepklosiren annectens\ three small ex- 

 ternal branchial filaments project from the single opercular aperture 

 on each side, and are long retained, if they be not permanent in that 

 remarkable osculant form between the osseous and cartilaginous fishes. 

 Some of the plagiostomous fishes are oviparous, but not as in the 

 majority of osseous fishes; a remarkable transposition in the periods 

 of the processes of fecundation and exclusion marks the distinction. 

 In the oviparous osseous fishes the ova are first excluded, then im- 

 pregnated : in the oviparous Plagiostomes impregnation is internal, 

 and precedes oviposition. The eggs are much fewer in number, but 

 their impregnation is more certain than in the scattered indiscriminate 

 act of spawning of the common fishes, where the countless numbers 

 of the ova seem to compensate for the chances that may intervene to 



* IliHlolijhi, i.xxv. Uatlike, cix. ; Leuckart, cxxiii. ; J. Davy, i.xxxi. 



■}• A. Tlioinpson, cxi. 



\ Jardine, cxxxv. ; aiul I'oteis, cxxxvi. 



