A system of obstetrics . iddle line belowand enveloped the notochord; ep, epiblast; ky, hypoblast; so, somatopleure; sp, splanchno-pleure; al, alimentary canal; pp, pleuroperitoneal cavity: the mesoblast cells lining itare already developing the epithelium of the future pleural and peritoneal cavities; wd,Wolffian duct; y, cleft between mesoblastic somite and lateral plate ; am, true amnion : thisis omitted in Fig. 65. changes in the relative sizes and positions of different parts of theembryo: this cleft also becomes obliterated at a later stage in develop-ment, and the lateral and vertebral
A system of obstetrics . iddle line belowand enveloped the notochord; ep, epiblast; ky, hypoblast; so, somatopleure; sp, splanchno-pleure; al, alimentary canal; pp, pleuroperitoneal cavity: the mesoblast cells lining itare already developing the epithelium of the future pleural and peritoneal cavities; wd,Wolffian duct; y, cleft between mesoblastic somite and lateral plate ; am, true amnion : thisis omitted in Fig. 65. changes in the relative sizes and positions of different parts of theembryo: this cleft also becomes obliterated at a later stage in develop-ment, and the lateral and vertebral plates again become continuousI Fig. 79). The unsegmented mesoblastic sheath of the notochord through itsupper extensions, or neural plates (vr, Fig. 69), provides a continuousenvelope for the neural canal. This condition gives place to the sec-ondary segmentation of the spinal column by which the vertebrae areseparated from one another; but the new segments are opposite the inter- 144 DEVELOPMENT OF THE EMBRYO. Fig. vols between the former} The mesoblast immediately around the noto-chord becomes cartilaginous at intervals, and forms a number of nod-ules which encroach on and constrict the notochord (Fig. 70). Eachnodule arises opposite one of the division lines (x, Fig. 65) between theoriginal somites, and is the rudiment of the centrum of a tissue between the nodules becomes fibrous and forms ultimatelyan intervertebral disk. Almost simultaneouslythe neural plates (vr} Fig. 69) become cartilag-inous at intervals corresponding to the rudi-ments of the vertebral bodies; and these carti-lages, joining the centra below and meeting overthe neural canal above, are the rudiments of theneural arches of the vertebras. The mesoblastof the neural ridge between the neural archesgives origin to the ligaments uniting them; theinner layer of the ridge nowhere becomes car-tilaginous, but develops into the perichondrium,periosteum, and dura mater. The modification of mesobla
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
