. The chordates. Chordata. Mammalia: Visceral System? 631 pc~. Zha ney at all, but connect directly into the hind end of the postcava (Fig 71E). Only arterial blood enters the kidneys. In sonic mammals, but not char- acteristic of the ("lass as a w hole, I here is a striking modification of the em- bryonic venous system. In all verte- brates the embryo develops a right and a left precaval vein (duct of Cuvier), each formed by union of the postcardinal, precardinal (jugu- lar), and subclavian of its side of the body, and both opening into the right auricle. In later embryonic stages of man
. The chordates. Chordata. Mammalia: Visceral System? 631 pc~. Zha ney at all, but connect directly into the hind end of the postcava (Fig 71E). Only arterial blood enters the kidneys. In sonic mammals, but not char- acteristic of the ("lass as a w hole, I here is a striking modification of the em- bryonic venous system. In all verte- brates the embryo develops a right and a left precaval vein (duct of Cuvier), each formed by union of the postcardinal, precardinal (jugu- lar), and subclavian of its side of the body, and both opening into the right auricle. In later embryonic stages of many mammals a large vein develops, extending transversely and just ante- rior to the heart, connecting the left precava, or the posterior end of the left precardinal (jugular), with the right precava. The result is that some blood from the left side flows across into the right precava (Fig. 479, left). In some mammals (sloths, anteaters and other "edentates"; whales; sea cows; carnivores; monkeys, apes, man), after the establishment of this connection, that part of the left pre- cava intervening between the trans- verse vein and the heart eventually undergoes complete atrophy so that all the blood carried by its former tributaries flows across into the right precava (Figs. TIE, 479, right). In the Australian duckbill and spiny anteater (both egg-laying mammals), in marsu- pials, and, with some exceptions, in insectivores, rodents, bats, and ungu- lates, both precaval veins persist al- though in some cases the cross connec- tion between them is established. The absence of the left precava in the adult mammal results in an asymmetry of the main venous trunks equaling that of the aortic trunks. A certain balance, or convenient spacing of the large vessels Fig. 479. Late embryonic stages in the development of the veins of a mammal. Ventral views. (Left) Right and left postcardinals and precardinals (ri) symmetrically developed and with a cross con- nection established betwe
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