in the Fccttis of Vertehrated Animals. 253 



The respiration, therefore, of the fcetus of the lizard, as well 

 as that of all oviparous animals higher in the scale, is entirely 

 performed by the two membranes to which we have just alluded, 

 viz. the sac of the yolk and the prolongation of the urinary 

 bladder or allantois *. 



Many lizards and serpents, however, are not truly oviparous, 

 but retain their ova till the development of the foetus has pjo- 

 ceeded some way, in general till the allantois becomes sufficiently 

 expanded to be fitted to carry on respiration ; and some serpents, 

 such as the Coluber berus, are almost entirely ovo-viviparous. 

 In this last it is curious to observe that the arterialization of the 

 foetal blood is effected, though by a simpler apparatus, in nearly 

 the same manner as that of mammalia, or truly viviparous ani- 

 mals. The allantois of the viper, after expanding so as to 

 enclose the foetus and yolk, comes into contact with a vascular 

 lining of the oviduct, and is closely united with it, so that the 

 venous blood of the foetus is exposed to the influence of the 

 oxygenized arterial blood of the parent. 



It is an interesting fact also, that in some of the Testudines, 

 the allantois, or at least a part of it, remains permanently in the 

 adult, and that the umbilical vein continues as in the batrachia 

 to carry off its blood to the liver. In the Testudo orbicularis, 

 according to Townson, water is introduced into this sac, and it 

 appears not improbable that the large urinary bladder, or per- 

 manent allantois of the turtles and of some serpents, serves as an 

 auxiliary in the function of respiration during the whole of life,*!*. 



Although respiration by means of gills is rendered unnec^^. 

 sary, by the perfect state of the allantois in these reptiles, yet it 

 appears, from some late observations, that at a period of the 

 development of these, as well as of all the more perfect animals 

 which have been examined, corresponding with that at which 

 the branchiae of fishes and batrachia begin to be formed, the 



• The connexion of the sac of the yolk with the intestine has been demonL 

 strated also in the Coluber natrix by Bojanus, Journal de Physique, 1829 ; aod 

 Dutrochet has shown very clearly the mode of development of the aliantoid 

 in the viper, in the Memoires de la Societe Medicale d'Emulation, torn. viii. 

 Several preparations in the College of Surgeons' Museum, London, illustrit^' 

 these facts extremely well, and some the sac of the yolk of the turtle. ^< 



