SPENCER, HERBERT, controversy with Weismann on polymorphism in insects, 125. Spermatogenesis, the formation of spermatozoa in the testis, 13. Spontaneous generation, 2. Stolon, a strand of tissue connecting the individuals of colonial animals, 46. STRASBURGER, the value of the nucleus in heredity, 13, 18. T Termites, polymorphism in, 125. Transfusion of blood, 75. Transplantation of bone, 73, 74. TREMBLEY, grafting of Hydra, 72. Triton, an amphibian, experiments on the egg by constriction, 64. Tubularia, experimental heteromorphoses, 51. Tunicata, a group of marine animals clad with a leathery tunic, 14. U Unicellular organisms, animals (protozoa) and plants (protophyta) with the simplest structure, each being a single cell: immortality of, 17; division doubling in, 40. Unit, definition of a biological, 30. V Vegetative affinity, 66 _et seq._ Vertebrates, regeneration of lost parts, 47. VOECHTING, experiments on grafting, 70; harmonic and disharmonic union, 70; on cells, 114, 116; on plasticity of plants, 117, 119; on grafting, 120. W WEISMANN and preformation, 8-10; caution against experiment, 12; sources of his theory, 20, 21; Hertwig's description of his theory, 22; absence of proof for differentiating division, 34; symmetry of egg and adult, 55; immortality of germ-cells, 17, 80, 82; germ-tracks, 83; doubling division, 102; controversy with Spencer, 125. Willow, reproduction from slips, 46. Wilson, separation of segmentation spheres of amphioxus egg, 60. WOLFF, _Theoria Generationis_, 4. Wounds, healing of, in relation to idioplasm, xii. Y Yolk, nutritive material stored in an egg-cell, xvi. THE END. BILLING AND SONS, PRINTERS, GUILDFORD. Heinemann's Scientific Handbooks. MANUAL OF BACTERIOLOGY. By A. B. GRIFFITHS, Ph.D., F.R.S. (Edin.), F.C.S. Crown 8vo., Illustrated, 7_s._ 6_d._ MANUAL OF ASSAYING GOLD, SILVER, COPPER, AND LEAD ORES. By WALTER LEE BROWN, B.Sc. Revised, Corrected, and considerably Enlarged, with a chapter on the Assaying of Fuel, etc. By A. B. GRIFFITHS, Ph.D., F.R.S. (Edin.), F.C.S. Crown 8vo., Illustrated, 7_s._ 6_d._ GEODESY. By J. HOWARD GORE. Crown 8vo., Illustrated, 5_s._ THE PHYSICAL PROPERTIES OF GASES. By ARTHUR L. KIMBALL, of the Johns Hopkins University. Crown 8vo., Illustrated, 5_s._ HEAT AS A FORM OF ENERGY. By Professor R. H. THURSTON, of Cornell University. Crown 8vo., Illustrated, 5_s._ LONDON: WILLIAM HEINEMANN, 21, BEDFORD STREET, W.C. End of Project Gutenberg's The Biological Problem of To-day, by Oscar Hertwig