The first principles of heredity; with 75 illustrations and diagrms . Fig. 18.—AlternationOF Generations inPlant-Lice. {From Geddes andThomson, The Evo-lution of Sex.) At the base is an indi-vidual arising froma fertilized egg-cell ;this gives origin par-thenogenetically to asuccession of genera-tions. At the top areshown the male andfemale forms, whichultimately the side an earlierreappearance ofsexual forms is sug-gested. 26 THE FIRST PRINCIPLES OF HEREDITY produces in its turn, either by division (Aurelia) or bybudding (Hydrapolype), once more the sexual free-movinggeneration. T
The first principles of heredity; with 75 illustrations and diagrms . Fig. 18.—AlternationOF Generations inPlant-Lice. {From Geddes andThomson, The Evo-lution of Sex.) At the base is an indi-vidual arising froma fertilized egg-cell ;this gives origin par-thenogenetically to asuccession of genera-tions. At the top areshown the male andfemale forms, whichultimately the side an earlierreappearance ofsexual forms is sug-gested. 26 THE FIRST PRINCIPLES OF HEREDITY produces in its turn, either by division (Aurelia) or bybudding (Hydrapolype), once more the sexual free-movinggeneration. The interpolation of asexual reproduction by sporesbetween sexual generations is a well-studied phenomenon. Fig. 19.—Alternation of Generations in the Common Jelly-fish (Aurelia). (From Plaeckel.) {From Geddes and Thomson, The Evolution of Sex.) 1, free-swimming embryo ; 2, embryo settled down ; 3 to 6, thedeveloping asexual stage ; 7 and 8, the formation of a pile ofindividuals ; 9, their liberation ; 10 and i ij the free-living sexualform. among lower plants. In the ordinary Fern we have thebig fern-leaf, the asexual plant, producing the familiarspores at the back. These develop when in suitable groundinto an inconspicuous green organism, with male and femalesex-cells. From the union of both arises once more thetall fern-plant. In the Moss we have a similar process, but REPRODUCTION 27 in this case the sexual plant is the more conspicuous higher flowering plants all exhibit a sort of alternationof generations, inasmuch as the plant first throws offbranches—, individuals arising by the asexual processof budding—which in their turn produce sexual here the sexua
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