Elements of human physiology / by Henry Power.

  • Henry Power
Date:
1888
Catalogue details

Licence: Public Domain Mark

Credit: Elements of human physiology / by Henry Power. Source: Wellcome Collection.

    457/528 (page 443)
    The Ovum, than the albumin floats up to near the surface. 6. The yellow yolk when boiled, or otherwise hardened, presents concentric markings, that are termed lialones, and it is then seen to be invested ex- ternally by a thin layer of the white yolk prolonged from the margin of the blastoderm. The yellow yolk is composed of small spheroids, which contain no nucleus, but are filled with minute highly refractile albuminous granules. The white yolk is also com- posed of spheroids, but these differ from the spheroids of the yellow yolk in being smaller and in containing a nucleus. Eggs which, like those of the fowl, consist of two pai'ts, one of which undergoes segmentation, whilst the other does not, are called meroblastic. Eggs which, like those of mammalia, undergo total segmentation, are termed holoblastic. Cliemical composition of ttie fowl's eg^g:*— The shell is composed of 4*15 per cent, of organic sub- stance, chiefly albumen, impregnated with calcium carbonate to the extent of 93*7 per cent., and contain- ing also small quantities of magnesium carbonate (1*39 per cent), calcium phosphate (0-76), and iron phosphate. The yolk consists of 47*1 per cent, of water, 15*6 per cent, of albumin, 31*3 per cent, of ether extract, chiefly fats, 4'8 per cent, of alcohol, and 0*96 per cent, of in- organic salts. Amongst the substances which have been isolated are : vitellin, which is the characteristic albuminous compound of the yolk ; nuclein, chiefly obtained from the white yolk; lecithin, glycerin, phosphoric acid, cholesterin, palmitin and olein, a pigment named lutein, glycogen, and grape sugar. Ovum of man.—The human ovum diflers in several particulars from that of the bird. It is extremely small, having a diameter of only about yl^^th of an inch. It has an external investment named the zona jjellucida (Fig. 30, b), which is of considerable thickness, and capable of resisting tolerably strong