. Outlines of zoology. Zoology. 456 STRUCTURE OF VERTEBRATA. Body cavity.—In Amphioxus a paired pouch grows out from the archenteron. Almost at once this becomes divided up on either side to form a series of small sacs, the cavities of which form ultimately the true body cavity or ccelom. According to Hertwig, this is in type the method of formation of the ccelom throughout the Vertebrata. In the other Vertebrates, owing to modified processes of development, probably first arising from the pre- sence of much yolk, solid cell masses grow out in place of hollow sacs, but the cavities which appea
. Outlines of zoology. Zoology. 456 STRUCTURE OF VERTEBRATA. Body cavity.—In Amphioxus a paired pouch grows out from the archenteron. Almost at once this becomes divided up on either side to form a series of small sacs, the cavities of which form ultimately the true body cavity or ccelom. According to Hertwig, this is in type the method of formation of the ccelom throughout the Vertebrata. In the other Vertebrates, owing to modified processes of development, probably first arising from the pre- sence of much yolk, solid cell masses grow out in place of hollow sacs, but the cavities which appear later, apparently by splitting of the cell mass, are in reality the retarded cavities of true ccelom-pouches. The body cavity may form part of one or all of the following systems :—(i) excretory, void- ing waste by abdominal pores or by nephrostomes ; (2) reproduc- tive, receiving the liberated genital elements; and (3) lym- phatic, receiving transudations from visceral and abdominal organs. It is probably never quite closed, but may communicate with the exterior by abdominal pores (or through nephrostomes). Fig. 194.—Transverse section through a Teleostean embryo (diagram- matic).—After Ziegler. , Spinal cord; N., notochord; ao., aorta ; , cardinal veins (united); , segmental duct; c, ccelom or pleuro- peritoneal cavity; , position of opening into the renal system, median vitelline vein; y., yolk; En., endoderm of gut; myotome. The dots represent mesenchyme cells; the little circles, blood corpuscles. Both occur together in some Elasmobranchs, but they are usually mutually exclusive. In the higher Teleostei, in some Saurians, and in Mammals, there are neither abdominal pores nor nephrostomes, but only openings (stomata) into the lymphatic system. Vascular system. — From Cyclostomata onwards the blood fluid contains red corpuscles, cells coloured with haemoglobin—a pigment which readily forms a loose union with oxygen, and bears it from
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