DEVELOPMENT AND LIFE-HISTORY.—The segmentation of the egg is holoblastic, and gives rise to a number of blastomeres, {107}which are for a time masked by the phenomenal activity of certain cells of extraneous origin, the "kalymmocytes," derived from the follicular epithelium surrounding the ovum. These follicular kalymmocytes migrate into the ovum, surround groups of blastomeres, and arrange themselves so as to reproduce the essential structure of the future embryo for which they form what may be termed a scaffolding or temporary support. After a time the blastomeres become active, proliferate rapidly, and finally press upon and absorb the kalymmocytes, and so eventually take their proper place in building up the organs. Some observers regard the kalymmocytes as being passive and nutritive only in function. [Illustration: FIG. 67.—Young solitary _Salpa democratica-mucronata_ attached to the parent by the placenta. _atr.ap_, Atrial aperture; _br_, dorsal lamina; _cil.gr_, dorsal tubercle; _ebl_, elaeoblast; _end_, endostyle; _n.gn_, nerve-ganglion; _oes_, oesophagus; _or.ap_, branchial aperture; _peric_, pericardium; _pl_, placenta; _rect_, intestine; _stol_, stolon; _stom_, stomach. (From Parker and Haswell, after Salensky.)] At an early period in the development a part of the surface of the embryo, on its ventral edge, becomes separated off, along with a part of the wall of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies, to form the "placenta" (Fig. 67, _pl_) in which the embryonic and maternal blood-streams circulate in close proximity, and so allow of the conveyance of nutriment to the developing embryo by means of large migrating placental cells. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up with oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67, _ebl_)—which is used up later in the development. Many suggestions have been made as to the homology and meaning of the elaeoblast; but it may now be regarded as most probable that it is reserve food-material associated with the disappearing rudiment of the {108}tail found in the larval condition of most Ascidians. The development is direct; and it may be said, then, that this young asexual (solitary) _Salpa_ differs from the corresponding form in the life-history of _Doliolum_ (Fig. 60, A) in that its tail is no longer a locomotory organ, but is represented by a nutritive mass, the elaeoblast, while the body, in place of being free, is attached by its ventral surface to a special organ of nutrition—the "placenta"—in connexion with the blood-stream of the parent.