252 Dr Allen Thomson on the Vascular System 



After the first half of foetal life has passed, the yolk sac is re- 

 moved from the inside of the chorion, and its place is supplied by 

 another vesicular membrane coming from the foetus, which gra- 

 dually expanding, entirely encompasses the foetus, and on which 

 the blood is now exposed, in order that it may undergo the 

 necessary respiratory changes. The sac of the yolk still remains, 

 however, covered by its network of vessels, which probably 

 serve to absorb the substance of the yolk for the nourishment of 

 the foetus, or being now less exposed to the air, perform some 

 subordinate part in respiration (Fig. 17, x y). 



The vesicular membrane which supplies the place of th^ 

 yolk sac as a respiratory organ, is the same part in a more ex- 

 panded condition, which becomes developed in the batrachian 

 tribes at the commencement of their aerial existence. According 

 to the observations of the authors already quoted, the allantois 

 of the Lacerta agilis appears, at the earliest period at which it 

 has been seen, in the form of a small vesicle hanging from the 

 lower part of the abdomen, projecting like a process from the 

 cloacal part of the intestine, and resembling in its form and 

 position the urinary bladder (See Fig. 19, Part I.). At this 

 time the allantois is covered by a network of minute vessels, 

 which become more obvious as the vesicle expands. Conti- 

 nuing to enlarge, it insinuates itself between the foetus and 

 the covering of the ovum (See Diagram, Fig. 18), and its 

 farther progress outwards being impeded, it expands laterally 

 till it envelopes the foetus in its amnios, and the yolk, with a 

 double layer of a vascular membrane (Fig. 17, z t>'). The 

 vessels distributed on this membrane are the umbilical arteries 

 and veins ; the outermost layer is the most vascular, and is ap- 

 plied close to the inner side of the envelope of the ^g^^ where it 

 receives the full influence of the air transmitted to it from with- 

 out. The allantois continues to act as a respiratory organ till 

 the foetus breaks the shell, or tears the covering of the Qg^^^ and 

 comes out fitted for receiving air into its lungs, and respiring 

 solely by these organs. The greater part of the allantois re- 

 mains in the Q^g^ in the Lacerta agilis, and the urinary bladder 

 of the adult is formed by a part at its root separated from it by 

 the urachus, on each side of which the umbilical vessels previ- 

 ously proceeded to gain the expanded part of the vesicle. 



