. The comparative anatomy of the domesticated animals. Veterinary anatomy. 1010 EMBRYOLOGY. unnecessary here to enter into a consideration of the very complex details of its structure and mode of formation. The above description shows the fundamental identity between the holoblastic. DIAGRAMMATIC SECTION OF A FOWL'S UNHATOHED EGG. I., Blastoderm; , white vitellus, consisting of a pyrit'orm central mass and a certain number of concentric layers; y., yellow vitellus, in alternate layers with the preceding; , vitelline membrane; x., layer of albuminous fluid surrounding the vitellus; w.,
. The comparative anatomy of the domesticated animals. Veterinary anatomy. 1010 EMBRYOLOGY. unnecessary here to enter into a consideration of the very complex details of its structure and mode of formation. The above description shows the fundamental identity between the holoblastic. DIAGRAMMATIC SECTION OF A FOWL'S UNHATOHED EGG. I., Blastoderm; , white vitellus, consisting of a pyrit'orm central mass and a certain number of concentric layers; y., yellow vitellus, in alternate layers with the preceding; , vitelline membrane; x., layer of albuminous fluid surrounding the vitellus; w., consistent albumen ; , chalazae; , air-chamber; , inner layer of the testaceous or shell membrane; , external layer of ditto; s., shell. Fig. 547. and meroblastic ova ; they only differ in the enormous predominance of the nutritive vitellus in the latter. This predominance has the mechanical effect 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 another 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 primitive trace—embryonal rudiment, or axile layer. The primitive trace is marked on the blastoderm by two dark streaks. Please note that these images are extracted from scanned page
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