Archive image from page 55 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( Primary mesoderm - § inner mass become converted into hollow vesicles by the appearance of cavities in their interiors (Fig. 29). When the above-mentioned changes have occurred the zygote consists of three spheres, one large and two small. The large sphere is bounded by the tropho- blast, and it contains the two small spheres and the jelly-like mass of primary mesoderm derived from the primary mesoderm segment of the inner mass (Fig. 29). The two small
Archive image from page 55 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( Primary mesoderm - § inner mass become converted into hollow vesicles by the appearance of cavities in their interiors (Fig. 29). When the above-mentioned changes have occurred the zygote consists of three spheres, one large and two small. The large sphere is bounded by the tropho- blast, and it contains the two small spheres and the jelly-like mass of primary mesoderm derived from the primary mesoderm segment of the inner mass (Fig. 29). The two small vesicles lie ex-centrically in the interior of the larger vesicle. The larger and more external of the two is the ecto-mesodermal vesicle. It is separated from the trophoblast, peripherally, and the entodermal vesicle, centrally, by the sur- rounding mesoderm. The early appearance of the mesoderm in the zygote and its insinuation at so early a period be- tween the ectoderm and the entoderm are peculiar- ities limited to the human subject. In most mammals the mesoderm does not appear until the embryonic area and its primitive streak are defined. The Embryonic Area.—The area where the two he in apposition with each other is the region of the zygote from which the embryo will be formed; it is called, therefore, the embryonic area, and at the time of its definition it consists of three layers, ectoderm, primary mesoderm, and entoderm. It is uncertain whether the mesoderm which is present in the area at this period takes part in the formation of the embryo or is replaced at a later period by mesoderm derived from the cells of the ecto-mesodermal vesicle; the latter certainly forms a large part of the mesoderm of the embryo. The Extra-Embryonic Ccelom.—The extra-embryonic ccelom is a space which appears as two clefts, one on either side of the embryonic area, in the primary mesoderm (Fig. 30). The clefts fuse together round the periphery of the embryonic area, and
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