99 Dr Allen Thomson on the Vascular System 



don, collected by Sir E. Home and Mr Clift (see sketch, Fig. 7)., 

 I could easily perceive that each of the filaments of which the 

 five bundles were composed consisted of a single fold or loop of 

 vessel, covered by a thin membranous layer prolonged from the 

 integuments. (Fig. 7, A.) • 



Development of the Respiratory Organs in the Foetus of Reptiles, 



1. Batrachia. — The development of the respiratory organs 

 in the foetus of these animals is peculiarly interesting on ac- 

 count of the transition which they undergo from an aquatic to 

 an aerial condition ; and the observation of the changes of 

 structure which take place during their transformation, ap* 

 pears to have illustrated, more clearly perhaps than that of any 

 other class of animals, the relations of the respiratory and vas- 

 cular organs to one another. 



Aquatic Salamander. — Shortly after the foetus or larva of the 

 Aquatic Salamander leaves the e^g, its blood is exposed to the in- 

 fluence of the surrounding water on the surface of the rudimenta- 

 ry intestine, or part corresponding with the sac of the yolk in 

 other animals. The vena cava returning from the tail, on arri- 

 ving at the posterior part of the intestine, gives off a large branch. 

 (Fig. 8, y'), which is joined by numerous small vessels spread 

 over the lower surface of the abdomen. The small vessels are 

 again united into one venous trunk (?;), situated near the 

 auricle of the heart, on the left side of the body, where they 

 pour into the heart (a) a large quantity of blood which passes 

 over the abdomen, along with that from other parts of the body. 



The liver (c) is formed in the batrachia, as in cartilaginous 

 fishes and the higher orders of animals, by the subdivision of 

 the venous trunk conveying the blood from the yolk to the heart. 



As the development of the foetus proceeds, its finny tail, as 

 well as the greater part of the integuments of the body, are 

 covered by minute ramifications of vessels, which must contribute 

 materially to aerate more perfectly the blood. As these rami- 

 fications become more numerous, the quantity of blood sent over 

 the abdomen becomes less, the liver (Fig. 8. c) increases in size, 

 the proper mesenteric vessels are formed, and the trunk of the 

 proper vena cava augments, and carries proportionally a greater 

 quantity of blood directly from the tail to the heart. At the 



