in the Fa'tus of Vertehrated Animals. 277 



3. In the saurian, ophidian, and cheionian reptiles, the ovum 

 being generally deposited in the medium which the animal per- 

 manently breathes *, an amnios or covering for the foetus is 

 formed, by means of which it is kept immersed in a fluid, till the 

 time when it is enabled to respire air ; respiration being carried 

 on, during the foetal lile of these reptiles, by the sac of the yolk 

 and allantoid vesicle highly developed. Some of these reptiles, 

 however, seem to be allied in some respects with the batrachia, 

 fts in them part or whole of the allantois remains permanent in 

 the adult state. 4. In birds, the application of a considerable 

 external heat is necessary to induce the proper respiratory al- 

 teration of the blood, which is exposed, as in the previous classy 

 pa the yolk and allantois, — membranes very highly developed in 

 birds. In these animals, a very small pedicle only of the allan- 

 tois, the urachus, remains in the foetus after birth. 5. In mam- 

 malia, again, the ovum being retained in the body of the mo^ 

 ther, the respiratory changes are effected by the intervention of 

 the maternal blood ; and another organ, formed by an exten- 

 sion of the umbilical vessels, is superadded in mammalia to the 

 yolk and allantoid, which exist as in birds. This is the pla- 

 centa, by means of which vessels, containing the venous blood 

 of the foetus, are brought closely into contact with the vessels 

 lining the uterus, and containing blood more highly aerated, 

 by which contact their respiration is effected. ./Bd9Wi.IIZ c 



XIII. It has also been shewn in the preceding relation, that 

 the gills are invariably formed on processes of the hyoid bone, 

 which are either permanent, as in fishes, or exist only during 

 the larval state, as in batrachia. The operculum is developed 

 fjpom the posterior part of the lower jaw, and this part, as well 

 as (the branchiostegous membrane, appears to be intimately con- 

 nected with the lingual bone or the lateral branches of the 

 hyoid. ) 



XIV. The observations related above, seem to shew that tb^ 

 lungs, though they receive their vessels from branchial arches; 

 which, during some period of foetal life, are distributed on gills 

 or analogous parts, cannot with accuracy be compared to these lat- 

 ter organs, as has been attempted. The lungs, it has been shewnj 



• Making exception in these, as well as other animals, of such as beccdne 

 developed iii the body of the parent. 



