The first principles of heredity; with 75 illustrations and diagrms . Ovum OF Sea-Urchin, Toxopneustes Lividus. (After Wilson.) zk, cell-body ; A, nucleus ; n, nucleolus. Below the ovum thespermatazoon {sp) of the same animal is drawn with the samemagnification. and, further, a nucleus, here called the germinal vesicle/which again contains the nucleolus and the chromatinsubstance. The size of the ovum varies considerably. It is oftenmicroscopical, as in the mammalian eggs, but can attain,on the other hand, enormous proportions, as in the case ofbirds. The essential part of the egg-cell, howeve
The first principles of heredity; with 75 illustrations and diagrms . Ovum OF Sea-Urchin, Toxopneustes Lividus. (After Wilson.) zk, cell-body ; A, nucleus ; n, nucleolus. Below the ovum thespermatazoon {sp) of the same animal is drawn with the samemagnification. and, further, a nucleus, here called the germinal vesicle/which again contains the nucleolus and the chromatinsubstance. The size of the ovum varies considerably. It is oftenmicroscopical, as in the mammalian eggs, but can attain,on the other hand, enormous proportions, as in the case ofbirds. The essential part of the egg-cell, howeverâthe 5 34 THE FIRST PRINCIPLES OF HEREDITY nucleusâis always small, generally visible only with theaid of the microscope, the remainder of the egg beingmade up by various extrinsic additions, but chiefly theyolk. The latter is material stored up within the egg toserve the forthcoming embryo as nourishment. We cansee how the large size of the birds egg, , is mostly madeup by the yolk, which lies around the very minute germinalvesicle. DM GD Bl WD EW KS CH. Fig. 23.âHens Egg : Diagrammatical Longitudinal Section. (After Allen Thomson.) [From Weismann, The Evolution Theory.) CH, chalaza ; DM, vitelline membrane ; GD, yellow yolk ; Bl, ger-minal disc with germinal vesicle; WD, white yolk ; EW, albumen ;KS, shell ; S, shell membrane ; LR, air-chamber. According to the disposition of the yolk within the egg,we distinguish four types of eggs, each of which is charac-terized, as w^e shall see later, by a typical method of seg-mentation. We have eggs with (i) diffuse yolk, wherethere is a small amount of yolk evenly distributed through-out the egg-cell, as in Invertebrates (Sponges, Corals, Star-fish, Worms, etc.), and Mammals ; (2) central yolk, theyolk being in the centre of the egg, as in Arthropods THE GERM-CELLS 35 (Crusters, Insects) ; (3) polar yolk, a large proportion ofyolk, accumulated chiefly in the lower half of the egg, as inAmphibians (Frog) ; and finally (4) predo
Size: 1882px × 1327px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1910
