(2138). While the above phenomena are in progress, another important system of vessels, provided for the respiration of the bird in ovo, is developed, and obliterated before the egg is hatched.

(2139). At about the period represented in fig. 374, the sides of the abdominal cavity, which is still open anteriorly, are occupied by transitory secreting organs, named corpora Wolfiana; these, apparently, are the rudiments of the genito-urinary system; and, to receive their secretion, a bladder is developed, called the allantoid sac, - a viscus which is moreover destined to play an important part in the economy of the embryo, 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 penetrates the egg-shell, it ultimately takes upon itself the respiratory function. When fully developed (fig. 376), it is covered with a rich network of arteries and veins (a, b) 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 condition, into the inferior cava, as does the umbilical vein of Mammalia.

Vessels of the allantois.

Fig. 377. Vessels of the allantois.

(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 to assume their function, by breathing the air that this vesicle contains. The circulation through the allantois then gradually diminishes, and it is slowly obliterated, until merely a ligamentous remnant, called the urachus, is left. In Reptiles, however, 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 Ovipara, 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, the allantoid apparatus is not developed. The egg in these lower tribes contains only the yelk and the cicatricula; it swells from absorbing the surrounding water, and the fetus is developed upon the surface of the yelk, - the latter, which, as in Birds, communicates with the intestine, being slowly received into the abdominal cavity.

Position of the chick in ovo.

Fig. 378. Position of the chick in ovo.

Viscera of mature chick.

Fig. 379. Viscera of mature chick.

(2142). The subsequent changes that occur in the circulatory system of a bird, namely, the obliteration of the foramen ovale and of the ductus arteriosi, whereby the pulmonary and systemic circulations become quite distinct, are similar to those which take place in the Mammiferous foetus, and will be described in the next chapter.