AVES BIRDS. 



629 



its principal respiratory organ. The allantois first makes its ap- 

 pearance as a delicate bag (Jig. 288, p), derived from the anterior 

 surface of the rectum, but Fiv 29 i 



it expands rapidly, and 

 soon occupies a very con- 

 siderable portion of the 

 interior of the egg (Jig. 

 289, c), until at last "it 

 lines nearly the whole ex- 

 tent of the mcmbrana 

 putaminis, and, becoming 

 thus extensively exposed 

 to the influence of the air 

 that penetrates the egg- 

 shell, it ultimately takes 

 upon itself the respiratory 

 function. When fully de- 

 veloped (Jig. 290), it is 

 covered with a rich net work 

 of arteries and veins (a, b) 

 spread upon its surface. 

 The arteries (fig. 291, a) 

 are derived from the com- 

 mon iliac trunks of the em- 

 bryo, and of course repre- 

 sent the umbilical arteries 

 of the human fetus ; the 

 vein enters the umbilicus, 



and, passing through the fissure of the liver, pours the blood, which 

 it returns from the allantois in an arterialized condition, into the 

 inferior cava, as does the umbilical vein of Mammalia. 



About the nineteenth day of incubation, the air-vessel at the 

 large extremity of the egg (Jig. 290, c) is ruptured, and the lungs 

 begin to assume their function, by breathing the air that this 

 vesicle contains. The circulation through the allantois then gra- 

 dually diminishes, and it is slowly obliterated, until merely a liga- 

 mentous remnant, called the urachus, is left. In Reptiles, how- 

 ever, as we have already seen, a portion of the allantoid bag re- 

 mains even in the adult creature (fig. 254, q); and in Birds 

 that compartment of the cloaca in which the genital and urinary 

 passages terminate are vestiges of the same organ. 



