Harmonic union in grafting, 70. Heteromorphosis, explanation of, 49; cases of, 51, 52; embryonic cases, 54. His, presence of foci in the germ, 13. Histogenous, producing microscopical characters, 20. Histology, study of the microscopical characters of cells and tissues, differentiation, 115. Hydatina, determination of sex, 5; temperature, 123. Hydra, regeneration in, 47; grafting of, 72. Hydromedusæ, a group of invertebrate animals, the typical members of which are branched colonies of polyps: Weismann's investigations on, viii, xii. Hypoblast. See Epiblast, xvi. Hypotrichous infusoria, a group of protozoa, 41. I _Ids_, hypothetical individual pieces, a number of which are supposed by Weismann to be present in the germplasm of every sexual cell, and each of which is supposed to contain the inherited material necessary for a complete new organism. It has been suggested that tiny beads seen within the chromosomes of a sexual cell are the _ids_, viii, 23, 33. Idioblasts, Hertwig's name for hypothetical ultimate units of living matter, 22, 82; the ultimate units of living matter, according to De Vries, 22. Idioplasm, as opposed to germplasm, which is the nuclear material of germ-cells; idioplasm is the nuclear material of tissue-cells, xi, 38. Immortality, definition of, 82; of germ-cells, ix; of unicellular organisms, 17; of germ-cells, 80. Individuality of cells, 115. Invagination, the infolding of a layer of cells, as, for instance, in the transformation of a blastosphere into a gastrula, xvii. Isotropism, explained in footnote, 33. K Karyokinesis, a complicated process of nuclear division, xiv. Katabolism, the formation of less complex chemical bodies by the agency of protoplasm, 86. L Labile, unstable, constantly changing, 38. LANDOIS, experiments on transfusion of blood, 75. LEIBNITZ, on immortality, 82. LOEB, on heteromorphoses, 49; on plasticity of animals, 117. M MAUPAS, experiments on sex of rotifers, 123. Melons, determination of sex by temperature, 124. Mesoblast, in the development of the coelomata, or three-layered multicellular animals; a third set of cells, the mesoblast, arises between the epiblast and hypoblast, xviii. Monsters, relation of, to division of egg-cell, 63. Mosaic theory of Roux, 56. Morphoplasm, the general protoplasm of a cell, 35. Multicellular organisms, those in which the body is composed of many cells, specialized in different directions; cell-division in, 43. Mus, experiments on grafting among mice and rats, 74. Myxomycetes, sometimes called 'slime fungi,' a group of low organisms, consisting of creeping masses of protoplasm with many nuclei, 33. N NAEGELI, biological units, 30; cross-fertilization and grafting compared, 69; heredity, 92; environment in development, 104; on plasticity of plants, 119; on specific characters of eggs, 134. Nais, regeneration in, 47. Notochord, formation of, from unusual cells, 117.