Section 38. But the allantois and yolk sac of the rabbit, though they correspond in development, differ entirely in function from the similar organs of the fowl. The yolk sac is of the very smallest nutritive value; instead of being the sole source of food, its contents scarcely avail the young rabbit at all as nourishment. Its presence in development is difficult to account for except on the supposition, that it was once of far greater importance. At an early stage, the outgrowing allantois, pushing in front of it the serous membrane, is closely applied to the lining of the mother's uterus. The maternal uterus and the embryonic allantois send out finger-like processes into each other which interlock, and the tissue between the abundant bloodvessels in them thins down to such an extent that nutritive material, peptones and carbohydrates, and oxygen also, diffuse freely through it from mother to foetus,* and carbon dioxide, water, and urea from the foetus to the mother. The structure thus formed by the union of the wall of the maternal uterus, allantois, and the intermediate structures is called the placenta. Through its intermediation, the young rabbit becomes, as it were, rooted and parasitic on the mother, and utilizes her organs for its own alimentation, respiration, and excretion. It gives off CO2, H2O, and urea, by the placenta, and it receives O and elaborated food material through the same organ. This is the better method that has superseded the yolk. * The embryo. 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.