ABSTRACT In Clytia johnstoni, mitosis is almost exclusively a function of the ectoderm cells; only very rarely does an endoderm cell divide. Thus new endoderm cells must be produced by the ectoderm cells. Cell divisions may occur anywhere in the stolonic ectoderm except for approximately the first 0·3 mm. from a growing tip. During the development of a hydranth mitoses are rare except for a local wave of ectodermal cell division contributing to the formation of the tentacles and buccal region. Growth only takes place in the first 6–12 mm. of a stolon. The ectoderm cells in the first 2–3 mm. move towards the tip; those on the ‘top’ of a stolon (i.e. away from the substratum) are concerned in the initiation of hydranths and those at the sides and bottom with stolon growth. In the remainder of this region ectoderm cells on the top of the stolon migrate into and form the ectoderm of the hydranths. The hydroplasm contains cells, mostly endoderm, which are derived from non-growing regions of the stolon and from regressing parts of the animal. During the ebb and flow of the hydroplasm, such cells are transported to, and adhere to the growing regions of stolons and to growing hydranths. The endoderm of hydranths is almost entirely supplied by this means. Cells differentiate into the various regions of the hydranth and stolon according to their positions relative to the growing tip.