in the Foetus of' Vertebrated Animals. 255 



lizard and tortoise. The anterior {)arts only of three other pairs 

 of branchial arches also remain to convey blood into the carotid, 

 subclavian and pulmonary arteries (See Fig. 19). 



The development of the lungs has been little attended to in 

 the higher reptiles. In lizards and tortoises the lungs are double, 

 and the pulmonary arteries are derived from the pulmonary arch 

 on each side (See Fig. 19). In serpents, on the other hand, the 

 lungs, as well as the pulmonary artery, are generally single ; but 

 in the Anguis fragilis, in the adult of which the lungs are double, 

 Rathke has observed, that in the foetus they are at first single ; 

 they consist of a mass of dense gelatinous substance, in which a 

 cavity is gradually formed ; in this animal the left is gradually 

 separated from the right lung in the progress of development, 

 and the cavities of the two lungs continue to communicate freely 

 with one another for some time. 



Development of the Respiratory Organs in Birds. 



For the development of the ova of birds, the application of 

 external heat, as well as the direct agency of air, is required, 

 and a respiratory change of the bio 3d of the foetus, comparatively 

 as extensive as that in the adult bird, seems to be necessary. 

 This change is produced by means of the same membranes, viz. 

 the sac of the yolk and allantois, as those on which the blood of 

 the fcetal lizard is exposed, and though these parts are consider- 

 ably more developed in birds, their relations, as well as the mode 

 in which they are supplied with vessels, are nearly the same as 

 in the higher reptiles. But the facts relative to the structure of 

 these membranes appear to be so well known, as to render it 

 unnecessary for me to enter into any detailed account of them. 



While the respiration of the foetal bird is almost entirely car- 

 ried on by the membranes of the yolk and allantois, the struc- 

 ture of the parietes of the neck and pharynx, as well as of the 

 adjoinng arteries, exhibits certain traces of branchiae similar to 

 those already alluded to in the embryo of the lizard. These ap- 

 pearances were first discovered by Rathke, and an account of 

 them was published by him in the Isis, in 1825 ; it is since this 

 time that the observations of this author have been extended, and 

 the same appearances discovered in the lizards, serpents, and 

 mammalia by himself, and by Huschke, Baer and others ; by 



