GEMMULE (from Lat. gemmula, little bud, diminutive of gemma, bud). In biology, (1) a mass of cells cut off from the parent for repro duction; (2) a hypothetical self-multiplying particle upon which inheritance depends.
(1) Among animals, gemmules are found in the groups of sponges and Polyzoa. In sponges, as winter approaches, numbers of the migratory cells form an aggregation in which two layers are eventually distinguishable. The central cells are loaded with yolk; the cells of the outer layer become club-shaped and arrange themselves in a sort of high epithelium. This layer of cells secretes a cuticular membrane around the inner mass of cells, and forms a layer of dumbbell shaped spicules close set in a radial fashion. The central cells are those from which the embryo is to arise next spring. The outer layer is protec tive. The gemmules thus constituted are set free when winter kills the sponge tissue. Next spring the inner cells grow and the bonds of the outer layer are broken. Such gemmules are found chiefly in fresh-water sponges, but within the last decade they have been found in marine sponges also. In the fresh-water Polyzoa the gemmules are of somewhat different character, and are called statoblasts. The statoblast arises in a special thread-like organ, the funicu lus, that is composed of ectoderm within and of mesoderm without. The ectodermal core pro liferates to form a hollow square, which later flattens and eventually produces the tough cutic ula by which the statoblast is covered. The outer mesodermal layer thickens, stores food material, and becomes enveloped by the ectoderm. In addition to the cuticula, which the ectodermal layer secretes, the statoblast is often provided with spines and a float which permits the stato blast to swim. In the spring the embryo de velops within the brown cuticula, bursts open this shell, and emerges to lay the foundations of a new colony. Both of the foregoing gemmules are devices for enabling the species to outlast the winter.
(2) The hypothetical material basis of inherit ance called gemmule by Darwin has been recog nized by one name or another by almost every philosophic writer in biology; other nearly or quite synonymous terms are the physiological units of Spencer, the bioblast of Beale, the pan gene of De Vries, the plasome of Wiesner, the micella of Nageli, the plastidule of Haeckel and Elssberg, the biophore of Weismann, somacule of Foster, idioblast of Hertweg, idiosome of Whitman, biogen of Verworn, and gemmule of Haacke. The hypothesis has arisen on account of the necessity of assuming a structure to proto plasm intermediate between the visible foam work and granules and the invisible molecules. The line of argument is briefly this: The qualities of the adult are inherent in the egg. and also in each of the cleavage spheres; each quality is rep resented by material particles, which divide when the cell divides; the particles are not molecules, for it is hardly conceivable that a molecule stands for a somatic quality; therefore there must be some sort of unit groups of interacting and internally associated molecules. Darwin's hypothesis (see PANGENESIS ) was that each cell threw off one or more gemmules; they floated in the blood to the germ-cells and became lodged in these cells. Galton tested this theory by trans planting the blood of one species of hare into a second. The progeny of the second was not in fluenced by the blood of the first species. Weis mann believed in no such migration of gemmules. The gemmules of the germ-cells receive no influx, of gemmules from outside by which their char acters might be changed; on the contrary, the ccmposition of the germ-cells is unchanged, says Weismann, except as a result of crossing or in ternal spontaneous modifications. See EMBRY OLOGY.