supporting lamella

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512 W. B. HARDY. powers, to be entirely free; and these are the elements which occur so characteristically in little groups, the cells of which so frequently betray signs of active proliferation. These histological facts, together with the absence of these free cells in other parts of the body and their peculiar relation to the gonophores, entitle us, I think, to regard them as preformed sexual elements. The Earliest Stages in the Formation of the Gonophore and its Relation to the Process of Budding in Myriothela. In early spring, and before sexual reproduction has taken place to any marked extent, specimens of Myriothela may be found which bear buds in various stages (PI. XXXVII, fig. 13). These appear to be always developed just at the junction of stolon and body. Once only have I met with a bud formed elsewhere, namely, in the lower tentacular region. This had, however, more the appearance of a permanent growth than of a bud to be cast off. The process of budding, so far as I have followed it, is a rather remarkable one. The first stage is a modification of the character of the ectoderm, which in the stolon and lower part of the body is composed of very long columnar cells, resembling the columnar cells of the blastostylar ectoderm in all particulars save in their inordinate length. Lying between the bases of these columnar cells are interstitial cells, characterised by the fact that they stain more deeply with picrocarmine. These cells appear to be partly nervous and partly concerned in the formation of nematocysts, which, curiously enough, are produced in limited number even under the thick and dense perisarc of the upper part of the foot. Where a bud is about to be formed the ectoderm-cells lose their defined characters, proliferate, and a bulging mass of amorphous tissue results. At the same time the thick supporting lamella becomes absorbed, and the endoderm-cells likewise proliferate and take on an amorphous character. The result is a kind of blastema in which the limits of ectoderm and
HISTOLOGY AND DEVELOPMENT OF MYRIOTHELA PHRYGTA. 513 endoderm are undistinguishable. This grows while at the same time its elements lose their distinctness and become highly charged with spherical masses of stored nutriment, resembling in many particulars the nutritive spheres of the general endoderm. As it grows it pushes the perisarc before it, and ultimately forms a rounded egg-like mass attached to the parent body by a short thick pedicle (fig. 13). From this the young Myriothela is developed (fig. 13). All connection with the body of the parent is lost at a very early period, almost before the bud has re-formed its ectoderm and endoderm and enteric cavity. It remains attached to the perisarc, however, by a sucker-like arrangement at the aboral pole until it is fully formed. As will be seen, the formation of a gonophore is, in its earliest stages, essentially similar to this method of budding. In other words, the gonophore is a true bud which, like the other buds, is derived from a blastema formed by a fusion of ectodermal and endodermal elements. The difference, however, lies in the fact that in the case of the gonophore bud, after it is a well-formed structure, a group of the primitive germ-cells make their way into it. The first stage in the growth of a gonophore is shown in fig. 8. The ectoderm of the gonophore-bearing region becomes thickened over a small surface, the increase in thickness being due largely to an accumulation of the primitive germ-cells, but partly to an increase in the cells carrying hyaline masses. At the same time nematocysts disappear in that region of the ectoderm, though they may occur in their usual profusion in close proximity. In the next stage the basement membrane is absorbed or ruptured (figs. 7 and 9), I cannot determine which, and a tongue of endodermcells pushes its way into the ectoderm and through the deepest layer. Thus the cluster of primitive germ-cells come to lie not on its apex, but, generally, asymmetrically disposed on one side. The removal of the supporting lamella is, I am inclined to think, mainly a process of solution, since scattered rounded VOL. XXXII, PART IV.—NEW SEE. M M
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