262 Dr Allen Thomson on the Vasnitar System 



communicating vessels (Figs. 29, 30 and 31, p p' J), forming 

 the ductus botalli of the bird, still retain a considerable size, till 

 the period when the inspiration of air into the lungs takes place, 

 when the whole of the blood entering the pulmonary arches 

 from the right ventricle, is carried into the pulmonary arteries, 

 and the branches communicating with the aorta are entirely 

 obliterated * 



Development of the Respiratory Organs in the Foetus of 

 Mammalia. 



As the ova of mammiferous animals are entirely developed in 

 the uterus, the blood of the fcetus is supplied with air entirely 

 through the medium of the parent, and the respiratory change 

 which it undergoes is comparatively much less perfect in these 

 animals than in birds. During the earlier periods of foetal de- 

 velopment in the mammalia, vascular membranes exist, ana- 

 logous to those which act as respiratory organs during the whole 

 of fcetal life in birds ; but these membranes seem to be capable 

 of producing the changes of respiration during a short time only, 

 and their place is soon supplied by another structure, by means 

 of which the bloodvessels of the foetus are brought into intimate 

 contact with those in the uterus of the mother. 



It has already been shown that the foetus of mammalia, like 

 that of other vertebrated animals, becomes developed on the 

 surface of the yolk, and that, while the rudiments of the foetal 

 organs are forming, the yolk is gradually surrounded by the 

 layers of the germinal membrane, upon which a vascular area 

 similar to that in birds is produced. The foetus of mammalia, 

 like the chick in ovo, lies with its left side towards the yolk. 

 The intestine of the foetus necessarily communicates with the 

 yolk-sac, as both these parts are formed by folds of the same 

 layers ; and the vascular network of the yolk is formed as in the 

 chick, by the omphalo-mesenteric arteries and veins. 



The yolk-sac differs very much in its relations in the different 

 families of the order Mammalia -f. In the common ruminating 



• See Baer's Entwickelungsgeschichte der Thiere, and the Repertoire 

 G^n^rale, torn. viii. 



• See Introduction to this Essay — See Baer de Ovi Mammalium et Ho- 

 minis genesi — Cuvier and Dutrochet in Memoires du Museum, vol. iii. 



