in the Fwtus of Vertcbrated Animals. 95 



by the union of the capillary vessels of this artery, conveys the 

 blood which passes over the yolk to the liver ; so that the distri- 

 bution of the omphalo-mesenteric vessels in the foetus of these 

 fishes resembles that in lizards, birds, and mammalia. 



As most of the fishes of this tribe are more or less ovo-vivipa- 

 rous, or retain their ova in the body during a longer or shorter 

 period after development begins, the blood of the foetus must 

 be made to undergo respiratory changes through the medium of 

 the fluids and membranes with which it is surrounded in the 

 oviduct. The very vascular membrane lining the oviducts in 

 some species of sharks which retain their ova during the whole 

 of development, is destined, without doubt, for the aeration of 

 the fluid surrounding the foetus ; and, according to SirE. Home*, 

 it would appear that the effect of this vascular membrane is 

 increased by the entrance of sea-water into the oviduct. The 

 apertures into the cavity of the peritoneum of these animals may 

 also allow the water to come into contact with the oviduct ; and, 

 in some of them, which retain their ova for a short time only 

 after development begins, apertures are found in the angles of 

 the horny covering of the egg^ through which a current of water 

 is permitted to pass. 



The yolk sac, so far as has yet been ascertained, is the only 

 foetal respiratory organ in osseous fishes ; while the blood con- 

 tinues to be exposed to the influence of the water on its surface, 

 the Gills or respiratory organs of the adult become developed. 

 According to Rathke, the rudiments of the gills may be percei- 

 ved in the embryo of the Blennius some little time after the cir- 

 culation of the blood has commenced. These organs appear at 

 first to be formed of five pairs of narrow plates, situated trans- 

 versely on the low^er side of the pharynx behind the mouth. 

 These plates, of which the four posterior only become developed 

 to form the perfect branchial hoops of this and other osseous 

 fishes, are at first composed of the same dense gelatinous sub- 

 stance as the rest of the embryo, and do not exhibit any traces of 

 vascularity. The same author has shewn, that the branchial 

 arteries begin to be formed soon after the appearance of the 

 rudimentary hoops, by the subdivision of the arterial vessel rising 

 from the bulb of the aorta (Fig. 1, b)^ on the lower part of the 



• See an interesting paper in Phil. Trans. 1810. 



