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On the Development of the Vascular System in the Fcetus of 

 Vertebrated Animals, Part II. By Allen Thomson, M. D. 

 late President of the Royal Medical Society. Communi- 

 cated by the Author. (Concluded from p. 111.) 



Development of the Respiratory Organs in the Ophidia^ 

 Chelonia and Sauria. 



In proceeding to consider the development of the respiratory 

 organs of the higher orders of reptiles, we pass from those ani- 

 mals which are aquatic, either during the whole or some period 

 of their existence, to those which are entirely aerial during foetal 

 and adult hfe. 



The ova of the serpents, turtles, and lizards, are deposited 

 and become developed in the same medium in which the adult 

 animal respires ; but in these animals, as well as in birds and 

 mammalia, a proper envelope for the foetus or amnios is formed 

 by the reflection of the serous layer of the germinal membrane *, 

 and the foetus is thus kept constantly immersed in a fluid till the 

 period when it begins to inspire air into the lungs. 



It is well known that the respiration of the foetal lizard, like 

 that of other animals, is first carried on by the distribution of 

 the blood over the surface of the sac of the yolk. In the former 

 part of this essay, it was stated that the observations of Dutrochet, 

 Emmert, Hochstetter and Baer, had shown that the embryo of 

 the lizard becomes developed on the surface of the yolk, and that 

 the blood and vessels are first produced in the form of a network, 

 on the surface of the area surrounding the foetus. This net- 

 work, like that in the ova of cartilaginous fishes, birds, and 

 mammalia, is composed of the minute ramifications of the 

 omphalo-mesenteric arteries, and of corresponding veins, which 

 carry to the vena portae the blood that has passed through the 

 network. The sac of the yolk, over which the vascular network 

 spreads itself more and more widely as development proceeds, 

 is at first situated near the internal surface of the shell, or other 

 covering of the ovum, and the blood contained in its vessels is 

 artcrialized by the transmission of oxygen from the air without. 

 • See Fig. 21 of amnios in birds. »». 



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