. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 602 ANATOMY OF VERTEBRATES. a minutely granular matter, with clear corpuscles, vitelline cells, and oil-particles: it projects from the surface of the yolk, fig. 422, a, and becomes transparent: the vitelline and oil-globules, aggregating at its base, buoy it up. The formation of two hyaline centres is followed by the cleavage of the germinal layer into two equal parts, ib. h, and these are next cleft at right angles into four, ib. c. In the Tench this occurred about half an hour after the rising of the germ-layer. Each
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. 602 ANATOMY OF VERTEBRATES. a minutely granular matter, with clear corpuscles, vitelline cells, and oil-particles: it projects from the surface of the yolk, fig. 422, a, and becomes transparent: the vitelline and oil-globules, aggregating at its base, buoy it up. The formation of two hyaline centres is followed by the cleavage of the germinal layer into two equal parts, ib. h, and these are next cleft at right angles into four, ib. c. In the Tench this occurred about half an hour after the rising of the germ-layer. Each of the four divisions undergoes subdivision, but irregularly, ib. d: further sub- division gives the surface a mulberry character, ib. e, and finally the parts are broken up to Such a dcffree of minuteness that the surface is again made smooth. The hyaline principle, which is the centre and cause of these successive divisions, is thus diffused through, or assimilated by, the whole germinal First steps In the devoinpemont of a Tench. layer, which has thereby become °^''^'' the ' germ-mass.' It now subsides to the form of a circular disc, separated by a layer of oil-globules from the yolk. The process occupies about three days in the Salmon, and from fifteen to twenty hours in the Pike:' before it is completed in the latter fish the yolk rotates within the ectosac.^ A cavity is formed in the centre of the germ-mass, which, as the mass subsides and extends over the yolk, is obliterated by the contact of the outer and inner layers. It clothes half the yolk by about the end of the third day, and when it covers two-thirds or more, the rotation ceases. The margin of the germ-mass encompassing the uninclosed part of the yolk is tumid. No rotation takes place in the ovum of the Perch,^ and the germ- mass incloses the whole vitellus, as in the Cyprinoids. The jjeripheral layer in the Pike begins to rise from the tumid margin of the germ-mass, as from a base, and extends, contracting, to
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Keywords: ., bookauthorowenrichard18041892, bookcentury1800, bookdecade1860
