of which has normally a different fate in subsequent development. What precisely each cell will become depends on its position with regard to the others. But each cell is capable of becoming more than it normally becomes: its potency is greater than its actual fate. If the normal course of development is interrupted, a cell, which would usually have given rise to a part of the skeleton, may give rise to a part of the alimentary canal. The cells of the developing embryo are autonomous. In the normal course of development most of the cells existing at the end of segmentation give rise to the “body” of the organism, undergoing differentiation as they so develop. But a few embryonic cells persist without structural modification throughout the development of the animal. They divide and grow and become greater in number, but remain unchanged in other respects. These cells become the essential reproductive organs, or gonads, of the adult animal--that is, the ovaries of the female and the testes of the male. In the females of the higher animals (the mammals, and perhaps some of the Arthropods) these cells only divide and grow during the early stages of development, and long before the beginning of adult life the number of ova in the gonads has become fixed. In all males, and in the females of most animals, however, the reproductive cells appear to be capable of unlimited multiplication. The essential cells of the gonads, the ovarian mother-cells or the sperm mother-cells, constitute the germ-plasm. In modern, speculative, biological literature the term germ-plasm is, however, restricted to the chromatic material in the nuclei of the reproductive cells, the cytoplasm being regarded as non-essential for the transmission of the hereditary qualities of the organism. It seems clear, however, that this distinction between the cytoplasm and the chromatic matter of the nucleus is not always a valid one, so that it is best to speak of the whole cell as constituting the germ-plasm. The embryonic cells, therefore, have different fates: some of them become transformed during development into the body or _soma_, and others remain unmodified throughout life as the _germ_. The soma enters into intimate relationships with the environment; it is affected by the vicissitudes of the latter; and it may actively respond to them. The germ-cells may possibly migrate through the body, perhaps, it has been suggested, developing fatally and irresponsibly into the mysterious, malignant tumours of adult life. Normally, however, they remain segregated in the reproductive glands, secluded from the outer environment. Their activities are inherent in themselves, are rhythmic, and become functional only on the assumption by the soma of the phase of sexual maturity. From the point of the species the soma is only the envelope of the germ-cells. It is affected by every change of the environment, and being usually cumulatively affected by the latter it becomes at