Section 39. in Its Later Development, the General Facts Already

enunciated with regard to the organs of frog and fowl hold, and where frog and fowl are stated to differ, the rabbit follows the fowl. In the circulation the left fourth vascular arch (second branchial) gives rise to the aortic arch; in the right the corresponding arch disappears, except so much of it as remains as the innominate artery. The azygos vein (Chapter 3) -is a vestige of- [is derived from] the right posterior cardinal sinus. Both pulmonary arteries in the rabbit are derived from the left sixth vascular arch (= fourth branchial). Compare Section 32. The allantois altogether disappears in the adult fowl; in the adult mammal a portion of its hollow stalk remains as the urinary bladder, and the point where it left the body is marked by the umbilicus or navel. The umbilical arteries become the small hypogastric arteries on either side of the urinary bladder. There is no trace of a pronephros at all in the rabbit.