. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. Fig. 75.—Diagrammatic transverse sections through a hypothetical mammal oSsperm (Had- don). A. The yelk of the primitive mammalian oosperm is now lost. B. Later stage; the non-embryonic epiblast has gi-owu over the embryonic area to form the covering cdls. ep, epiblast of embryo ; ep\ epiblast of yelk-sac ; Ay, primitive hypoblast; y. s, yelk-sac, or blastodermic vesicle. blood-vessels that derive the food-supplies, which replenish the blood as it
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. Fig. 75.—Diagrammatic transverse sections through a hypothetical mammal oSsperm (Had- don). A. The yelk of the primitive mammalian oosperm is now lost. B. Later stage; the non-embryonic epiblast has gi-owu over the embryonic area to form the covering cdls. ep, epiblast of embryo ; ep\ epiblast of yelk-sac ; Ay, primitive hypoblast; y. s, yelk-sac, or blastodermic vesicle. blood-vessels that derive the food-supplies, which replenish the blood as it is exhausted, from the hypoblast of the area opaca. The first indications of future structural outlines in the embryo is the formation of the primitive streak, an opaque band in the long diameter of the pellu- cid area, opaque in consequence of cell accummulation in that re- gion. Very soon a groove {primi- tive groove) extends throughout this band, which gradually occu- pies a more central position. The relative thickness of the several parts and the arrangement of cells may be gathered from Fig. 76. These structures are only tempo- rary, and those that replace them will be described subsequently. We have thus far spoken of cells as being arranged into epi- blast, hypoblast, and mesoblast. The origin of the first two has been sufficiently indicated. The mesoblast forms, the intermediate germinal layer, and is derived from the primitive hypoblast, which differentiates into a stratum of flattened cells, situated below the others, and constituting the later hypoblast, and in-. FiG. 76.—Surface view of pellucid area of blastoderm of eighteen hours (Foster and Balfour), if, medullary folds; mc, med- yllary groove ; pr, primitive 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 Mills, Wesley, 1847-191
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