. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERiVAL ANA TOA/Y. 69 These are the coelomic openings, or nephrostomes, of the tubules. At their outer ends most of them open directly into a longitudinal duct, the pronep/inc duct, which extends backwards to the cloaca. The most anterior tubules, however, tend to fuse to- gether at their outer ex- tremities, before reaching the common duct. Corre- sponding to each tubule there is a short artery- growing out from the dor- sal aorta, and abutting with its blind end against the portion of the body-cavity int

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. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERiVAL ANA TOA/Y. 69 These are the coelomic openings, or nephrostomes, of the tubules. At their outer ends most of them open directly into a longitudinal duct, the pronep/inc duct, which extends backwards to the cloaca. The most anterior tubules, however, tend to fuse to- gether at their outer ex- tremities, before reaching the common duct. Corre- sponding to each tubule there is a short artery- growing out from the dor- sal aorta, and abutting with its blind end against the portion of the body-cavity into which the innermost nephrostomes open. Later on these coecal outgrowths from the dorsal aorta develop a vascular network at their free ends, and so produce a series of glomeruli. If, now, we inquire into the mode of development of such a pronephric sys- tem as the one above de- scribed, we find that its component tubules arise as a series of knob-like seg- mental outgrowths from the outer or somatic layer of the peritoneum at the base of the segmented portion of the primitive body-cavity.. Fig. 33. — Schematic transverse section through a Selachian embryo in the region of the pronephros. (After VAN WlJHE.) The dotted Hne drawn across the section indicates the plane of division between the upper segmented and the lower unseg- mented portions of the primitive body-cavity (proccelom). my. Myotome or myomere. nis. Mesomere or nephrotome. /. Prone- phric outgrowth, sp. Unsegmented bod'- cavitv or splanchnoccel. sc. Sclerotome. n. Nerve-tube. .://. Notochord. ao. Dor- sal aorta, al. Digestive tube.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Willey, Arthur, 1867-. New York, London, Macmillan