. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. Fig. 547. DIAGRAMMATIC SECTION OF A FOWL'S UNHATCHED EGG. hJ., Blastoderm; , white vitellus, consisting «{ a pyritorm central mass and a certain number of concentric layers; y., yellow vitellus, in alternate layers with the preceding; , vitelline membrane; x., laver of albuminous fluid surrounding the vitellus; w., consistent albumen ; , chalaza; , iiir-chamber; , inner layer of the testaceous or shell membrane; , external layer of ditto; s., shell. and meroblastic ova ; they only diff
. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. Fig. 547. DIAGRAMMATIC SECTION OF A FOWL'S UNHATCHED EGG. hJ., Blastoderm; , white vitellus, consisting «{ a pyritorm central mass and a certain number of concentric layers; y., yellow vitellus, in alternate layers with the preceding; , vitelline membrane; x., laver of albuminous fluid surrounding the vitellus; w., consistent albumen ; , chalaza; , iiir-chamber; , inner layer of the testaceous or shell membrane; , external layer of ditto; s., shell. and meroblastic ova ; they only differ in the enormous predominance of the nutritive vitellus in the latter. This predominance has the mechanical efPect of localizing the phenomena of segmentation to one pole of the egg—that in which is the plastic vitellus ; and we have seen that this is com- pleted at the moment of laying. The blastoderm that issues from this centre of proliferation necessarily assumes the shape of a disc—the germinal disc; then of a spherical cap lying on the yolk by its concavity, and tending to em- brace it by the progressive extension of its circular border. This investment is only com- pleted towards the sixth day of incubation, and the blastoderm is therefore late in assuming the vesicular form. Notwithstanding differences of the same kind, arising from anotlier manner of proceeding than that observed in Mammals, the successive formation of the germinal area, and its doubling into an opaque and a trans- parent zone, take place. The appearance of the embryo is manifested by a temporary formation known as the primitire trace—embryonal rudiment, or axile layer. The primitive trace is marked on the blastoderm by two dark streaks. AREA PELLUCIDA IN THE VERY EARLY BLASTODERM OF THK CHICK, SHOWING THE PRIMITIVE LINE AT ITS FIRST APPEARANCE. , primitive trace ; ap, transparent or pellucid area; , opaque Please note that these images are extracted from scanned page images tha
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