The first principles of heredity; with 75 illustrations and diagrms . D Fig. 24,—Ovum and DispositionOF Yolk. (After Thomson.) A, ovum with diffuse yolk (sponge) ;B, ovum with central yolk (cray-fish) ; C, ovum with polar yolk(frog) ; D, ovum with predomin-ant yolk (bird). Fig. 25.—Spermatozoon.(After Wilson.) {From Weismann, TheEvolution Theory.) sp, point; 77, nucleus ; c,centrosome; m, middle-piece ; aXy tail. where nearly the whole egg is taken up by the yolk excepta tiny part at the upper pole of it, as in Fishes, Reptiles,and Birds. 36 THE FIRST PRINCIPLES OF HEREDITY (c) Spermatozoon. T
The first principles of heredity; with 75 illustrations and diagrms . D Fig. 24,—Ovum and DispositionOF Yolk. (After Thomson.) A, ovum with diffuse yolk (sponge) ;B, ovum with central yolk (cray-fish) ; C, ovum with polar yolk(frog) ; D, ovum with predomin-ant yolk (bird). Fig. 25.—Spermatozoon.(After Wilson.) {From Weismann, TheEvolution Theory.) sp, point; 77, nucleus ; c,centrosome; m, middle-piece ; aXy tail. where nearly the whole egg is taken up by the yolk excepta tiny part at the upper pole of it, as in Fishes, Reptiles,and Birds. 36 THE FIRST PRINCIPLES OF HEREDITY (c) Spermatozoon. The Spermatozoon is, as a rule, very much smaller thanthe corresponding ovum. Its form is adapted to itsfunction, for it actively seeks and penetrates the ovum. The typical spermatozoon—as, , in man—consists ofa small pointed head, composed nearly entirely of thenucleus, a middle piece, containing the centrosome, and along contractile tail, by means of which the spermatozoon. Fig. 26.—Spermatozoa. (From Geddes and Thomson, The Evolution of Sex.) a, crayfish ; b, lobster ; c, crab ; d, ascarid ; e, water-flea —moina ;f, man ; §?, ray ; h, rat; i, guinea-pig ; k, beetle—immaturestage ; /, sponge. effects its rapid undulatory movements. The shape inother cases may differ, be starlike or clublike, but theactive amoeboid movements are an essential feature of it. III.—MA TURA TION. Both germ-cells, ovum as well as spermatozoon, possessthe same number of chromosomes as the ordinary body-cells, which, as has already been remarked, is constant for THE GERM-CELLS 37 any given species. Before, however, the germ-cells areready to unite in the act of fertilization, they undergocertain changes, which have the effect of leaving them withonly half the number of chromosomes they had process in the ovum is called its maturation,while the same end is achieved for the spermatozoon duringits development—, during the process of spermato-genesis.
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