CIO •« ANATOMY OF VERTEBRATES. 



from ramifications of a branch of the mesenteric artery, and the 

 blood is returned to the mesenteric vein. Hunter's preparations 

 of the embryo Carcharias (No. 1061), Scyllium (No. 3250), 

 Spinax (No. .3255), and Alopias (No. 3261) ', demonstrate another 

 foetal j)ecullarlty which later researches ^ have shown to be pro- 

 bably common to all Plagiostomes, viz. the external fringe of 

 filaments developed from the branchial surfaces, b : a tuft extends 

 out of each aperture, and even from the spiracula, a, in the genera 

 with those accessory openings. Each filament contains a single ca- 

 pillary loop : ' they disapjiear early, being removed by absorption. 

 The last remnants may be seen in the preparation of the fcetal 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 

 '*-*' react on the streams of water admitted into the 



egg by the apertures, fig. 426, c. In the ovo- 

 viviparous Sharks the size and position of the 

 cloacal apertures of the uteri would seem adapted 

 to allow free ingress of sea-water; so that, 

 whilst the vitellicle, ib. i, administers to the nutri- 

 ment of the embryo, a, the external branchia; may 

 perform the respiratoiy function. In the smooth 

 Emissole {Mustclus levis), vascular cotyledons are 

 developed from the vitelline (omphalo-mesen- 

 teric) capillaries, which are firmly connected to 

 the viterine cotyledons ; so that here the vitel- 

 licle, like a true placenta, may perform both the 

 nutrient and respiratory functions : the external 

 branchiaj disappear some time before the exclusion of the embryo 

 and the aljsorption of the yolk. In the Lcpidosircn annectms^ 

 three small external branchial filaments project from the single 

 opercular aperture on each side, and are long retained. 



Some of the Plagiostomons Fishes are oviparous, but not as in 

 the majority of Osseous Fishes ; a remarkable transposition in the 

 times of the processes of fecundation and exclusion marks the 

 distinction. In the oviparous Osseous Fishes the ova are first 

 excluded, then impregnated: in the oviparous Plagiostomes im- 

 pregnation is internal, and precedes oviposition. The eggs arc 

 much fewer in nmuber, but their impregnation is more certain 

 than in the scattered indiscriminate act of spawning of the 

 Osseous Fishes, where the countless numbers of the ova seem to 



' XX. vols. ii. and v. 



- Kiuloljihi, Lxxvi. ; Ratlikc, cxi. ; Lcuckart, cxxv. ; J. Davy, lxxxit. 



' A. Thompson, cxj. ' xx. vol. v, » Jaraiuc, cxxxv.j Peters, cxxxvi. 



