Nucleus, a specialized portion of the protoplasm of cells, different in chemical and physical properties (see Chromatin, Chromosomes), as the bearer of heredity, 19. NUSSBAUM, views on origin of germ-cells, 17. Nutrition, influence of, on development, 2. O OLLIER, bone-grafting, 73. Ontogeny, the development of an individual from the egg upwards, 9. Osteoblasts, cells which are the active agents in bone-formation, 73. Ovogenesis, the formation of egg-cells in the ovary, 13. P Pangenesis, Darwin's provisional hypothesis, that the sexual cells were composed of minute particles (gemmules), given off by all the cells of the body, 21. Periosteum, a cellular sheath of bones, 73. Physiological units, Herbert Spencer's name for hypothetical ultimate units of living matter, 22. Pistachio, influence of temperature on, 121. Plant-cells, mode of growth, 110. Plasomes, Hertwig's name for theoretical units of protoplasm, 32. Plasticity of plant tissues, 117, 119, 120. Pluteus, a free-swimming larval stage in the development of echinoderms, 54. Podophrya, reproduction of, 41. Polymorphism, the appearance of the same species in several different forms in ants and social insects, 125. PONFICK, on transfusion of blood, 75. Preformation, identical with the original meaning of evolution, which see. Prothallus, the leaf-shaped green organism that grows from the spore of a fern and produces sexual organs, 49. Pseudopodia, extensions of protoplasm beyond the general contour of the cell, 41. R Radiolaria, a group of protozoa, 44. Regeneration in plants and animals, 45, 47. Rhipsalis grafted on Opuntia, 71. ROUX, contrast between epigenesis and evolution, 6; mosaic theory of, 56. Rudiment, used here as a translation for the word _anlage_, which means the first plotting-out or beginning of a living structure. Darwin showed that rudimentary organs in adult creatures were for the most part vestiges of organs that had lost their use. In this treatise 'rudiment' is applied to an organ or structure in its incipient condition, whether that incipient state be visible in a young embryo, or a hypothetical structure in the germplasm, 6; latent rudiments, 37. S SACHS, on cells, 114; on reaction and protoplasm, 133. Salix purpurea, reproduction from galls, 51. SCHMITT, bone-grafting, 74. Segmentation, the early division of a developing egg, xvii. Segmentation spheres, the cells resulting from the early divisions of a developing egg, separation of, by Wilson and Driesch, 60. Segmentation cavity. See Blastosphere. Sex, determination of, by temperature, 123, 124. Sexual cells (spermatozoa in male, ova or egg-cells in female), the nucleated pieces of protoplasm which are the starting-point of the new generation in sexual reproduction, origin of, 18. Soma, the body of a plant or animal as contrasted with the reproductive cells contained within it, 45. Somatic cells, the cells of the soma; mortality of, 17.