DEVELOPMENT OF THE EESPIEATOEY APPAEATUS. 



751 



tion, a bladder is developed, called the allantoid sac, a viscus which is 

 moreover destined to play an important part in the economy of the em- 

 bryo, and soon becomes its principal respiratory organ. The allantois 

 first makes its appearance as a delicate bag (fig. 374, p}, derived from the 

 anterior surface of the rectum ; but it expands rapidly, and soon occupies 

 a very considerable portion of the interior of the egg (fig. 375, c), until 

 at last it lines nearly the whole extent of the membrana putaminis, and 

 becoming thus extensively exposed to the influence of the air that pene- 

 trates the egg-shell, it ultimately takes upon itself the respiratory func- 

 tion. When fully developed (fig. 376), it is covered with a rich network 

 of arteries and veins (a, 6) spread upon its surface. The arteries (fig. 

 377, a) are derived from the common iliac trunks of the embryo, and 

 of course represent the umbilical arteries of the human foetus ; 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 condi- 

 tion, into the inferior cava, as does the umbilical vein of Mammalia. 



(2140.) About the nineteenth day of incubation, the air-vessel at the 

 large extremity of the egg (fig. 376, c) is ruptured, and the lungs begin 



Fig. 378. 



to assume their function, by 

 breathing the air that this 

 vesicle contains. The circu- 

 lation through the allantois 

 then gradually diminishes, 

 and it is slowly obliterated, 

 until merely a ligamentous 

 remnant, called the urachus, 

 is left. In Reptiles, how- 

 ever, as we have already seen, 

 a portion of the allantoid bag 

 remains even in the adult 

 creature (fig. 340, q) ; and in 

 Birds, in that compartment 

 of the cloaca in which the 

 genital and urinary passages 

 terminate, are vestiges of the 

 same organ. 



(2141.) Although the above 

 description is intended to give 



a general view of the process of oviparous generation in its most perfect 

 and consequently most complex form, the reader, in applying it to the 

 development of the ovum in the inferior OVIPAHA, must bear in mind 

 the following important differences: 1st, That in the air-breathing 

 REPTILIA the white of the egg is almost, if not entirely, wanting ; but 

 the other phenomena are similar to those witnessed in the Bird. 2ndly, 

 That in FISHES not only is there no white formed, but, for obvious reasons, 



Position of the chick in ovo. 



