A system of obstetrics . long been a diffi-culty, but now that the Wolffian duct has been found to be epiblastic inorigin, it is easy to conceive that in primitive vertebrates the segmentaltubes may have opened into a groove on the exterior, and that thisgroove may, much as the medullary groove, have closed over and beenseparated from the external epiblast: in this condition we find it inFig. 131; and as development proceeds it becomes more and moreimbedded in The posterior end of this primary segmentalduct opens at first into the hind-gut, and later into the urogenital sinus(p. 19
A system of obstetrics . long been a diffi-culty, but now that the Wolffian duct has been found to be epiblastic inorigin, it is easy to conceive that in primitive vertebrates the segmentaltubes may have opened into a groove on the exterior, and that thisgroove may, much as the medullary groove, have closed over and beenseparated from the external epiblast: in this condition we find it inFig. 131; and as development proceeds it becomes more and moreimbedded in The posterior end of this primary segmentalduct opens at first into the hind-gut, and later into the urogenital sinus(p. 194). As development proceeds the Wolffian duct becomes buried deeper(wd, Figs. 79, 81) in the mesoblast. Meanwhile, the pleuroperitonealcavity expands, and the mesoblast cells which lie ou its mesial sidemultiply rapidly and give rise to a projection which extends into theccelome, especially in the dorsal region of the embryo. This projec-tion is part of the intermediate cell-mass. At this stage segmental tubes Fig. Diagram of Cross-section of Embryo, shortly after the formation of the intermediate cell-mass,icm: the peritoneal cells on its surface are seen to have become enlarged; ao, aorta; ytWolffian duct, now buried deep in the mesoblast; z, duct of Miiller: on the left side severalWolffian tubules are shown in section; al, alimentary canal; am, amnion; pp, coelome. begin to burrow-in from the body-cavity and to connect with the Wolff-ian duct. They are the Wolffian tubules, or the tubules of the meso-nephros: subsequently they lose their openings into the pleuroperitonealcavity, which indeed the posterior tubules never possess. In birds and mammals the pronephros is very imperfectly developed,and lasts only for a very short period. Its duct, however, becomeswell marked on each side, and runs back parallel to the Wolffian duct(Fig. 133), to open into the cloaca near the root of the allantois. 1 See Haddon: loc. cit., p. 250. DEVELOPMENT OF ORGANS AND SYSTEMS. 193 The p
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
