Morris's human anatomy : a complete systematic treatise by English and American authors / ed. by C.M. Jackson eleven hundred and eighty two illustrations, three hundred and fifty eight printed in colors.

  • Sir Henry Morris, 1st Baronet
Date:
[1914], [©1914]
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Licence: Public Domain Mark

Credit: Morris's human anatomy : a complete systematic treatise by English and American authors / ed. by C.M. Jackson eleven hundred and eighty two illustrations, three hundred and fifty eight printed in colors. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    ■cavity; the deeper (D) is the cavity of the yolk-sac; while between them is a plate of cells forming the embryonic disc. The embryonic disc (figs. 9 and 10) contains three layers of cells,—the fundamental germ layers,—ectoderm (Ec), endoderm (En), &n.A mesoderm. The germ layers of the embryonic disc are of prime importance in the development of the body. From the ectoderm, which hes next to the amniotic cavity and represents the upper (later outer) germ layer, are derived the epidermis and the entire nervous system. From the ■endoderm, which hes next to the yolk-sac, and represents the lower (later inner) germ layer, is derived the epithehal lining of the digestive mucosa and its derivatives. From the mesoderm, or middle germ layer, is differentiated the remainder of the body, including the skeletal and sup- porting tissues, vascular system, muscle and most of the urogenital organs. The germ layers also extend beyond the embryonic disc, as shown in figs. 9 and 10. The yolk-sac is made up of a lining of endoderm and an outer layer of mesoderm. The amnion, which Fig. 10.—Diagram op Section of a Mammalian Ovum showing the Embryonic Disc, Amniotic Cavity and the Germ Layers. Endoderm later becomes separated from the chorion, is composed of mesoderm lined by endoderm. The outer cell layers form the chorion, which likewise shows two layers, the outermost of which (trophoblast) is ectoderm, the inner, mesoderm. In fig. 10 the chorion is beginning to send out root-like projections (villi) which invade the uterine mucosa. It is thus noteworthy that of the cells']derived from the ovum relatively only a few—those of the embryonic disc—enter directly into the formation of the body. The yolk-sac, a rudimentary organ of phylogenetic significance, is later chiefly absorbed, although the proximal portion may enter slightly into the formation of the intestinal wall. The amnion is a protective membrane, while the chorion forms the foetal part of the placenta. Development of the embryonic disc.—When first formed, the surface of the embryonic disc shows no trace of differentiation. A slightly later but still comparatively early stage in its development is shown in fig. 11. It is here Fig. 11.-—Model Showing the Embryonic Disc from an Embryo 1.17 mm. In Length. Viewed from above and laterally, the roof of the amniotic cavity having been removed, n, primitive pit (neurenteric canal), pg, primitive groove, mg, neural groove, b, body-stalk. (McMurrich. from Frassi.) viewed from above, the amnion having been removed. The disc is an elliptical plate, whose long axis represents the mid-line of the embryo. Near the center is a small rounded depression, the primitive pit. Extending backward (toward the tail end of the embryo) from this is a dark line, the 'primitive streak, corresponding to a groove, the primitive groove. Extending forward from the primitive pit is an indistinct wide shallow groove, the neural groove. At an earlier stage, the primitive streak extends further forward, possibly to the anterior end of the embryonic disc (Spee). The primitive streak and groove probably con-espond to the