. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. EARLY EMBHYONIC DEVELOPMENTS. 1007 The yolk is formed of a peripheral hiyer of clear transparent cells—the erfodfrm, or ('pibi(isf—V\u\ui>; the vitelline membrane, and emliraeiu^ a central mulberry mass {monila) of polyhedral and granular cells—the endodcnn, or hijpo- blasf. The ectoderm shows at one of its points an aperture—the blt/.sfopore, or mu/s of Rusconi—into which penetrates and closes it, but without overlapping it, a prolongation of the endoderm—the rork of Erker. At this phase in its evolu- tion,
. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. EARLY EMBHYONIC DEVELOPMENTS. 1007 The yolk is formed of a peripheral hiyer of clear transparent cells—the erfodfrm, or ('pibi(isf—V\u\ui>; the vitelline membrane, and emliraeiu^ a central mulberry mass {monila) of polyhedral and granular cells—the endodcnn, or hijpo- blasf. The ectoderm shows at one of its points an aperture—the blt/.sfopore, or mu/s of Rusconi—into which penetrates and closes it, but without overlapping it, a prolongation of the endoderm—the rork of Erker. At this phase in its evolu- tion, the yolk constitutes what is tinned in comparative embryology, the metU' gnstrula. 2. Passage of the Metagastrula to the Uterus, and Formation of THE Vesicle and Blastodermic Layers.—As soon as the metagaslrula has reached the uterus, it commences to be transformed into a clear and transparent vesicle, which grows rapidly, attaining in four or live days a diameter of from s to 9 mm., and constituting the h/asfodermic vesirlP, or blastoderm. The appearance of the blastoderm is marked by the appearance of a fissure that separates the ectoderm from the "ndoderm, leaving them only adherent at a point corresponding to the blasto- pore, which was already formed Fig. 544. towards the end of the third day. Owing to the pressure of the fluid that fills it and tends to ac- cumulate, the ectoderm becomes distended and fissures; its cells multiply, and it flattens so as to embrace a layer that, towards the ninetieth hour, attains from "If) to '17 mm. The endoderm, pressed upon by the fluid, is deformed and spread over a point beneath the ectoderm, where it becomes the (/rrafrodisc. The vesicle, formed at this part of two superposed layers, is still monodermic everywhere else (Fig. 544); nevertheless, the cells of the gastrodisc change in charac- ter, becoming flattened, and in an ovum of from 105 to 115 hours, having a diameter of from "0 to 2 mm., they form an
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