supporting lamella

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510 W. B. HARDY. characterised by the possession of an exceedingly distinct nucleolus. The smallest of these cells are generally isolated (fig. 6), tnd each consists of a nucleus surrounded by a delicate pellicle of exceedingly finely granular protoplasm. The whole cell may be only 5 m in diameter. They form the distinctive feature of the ectoderm of the proximal region of the blastostyles, and, by their number, give to it its great thickness, What part they play we shall see later. The remaining constituents of the proximal ectoderm as yet unnoticed are its nervous and muscular elements. These I propose to mention very briefly, for they lie to a certain extent outside the limits of the present paper. One of the most striking features of osmic acid preparations, whether sections or teased, are tufta of branching filaments with curious deeplystaining matter disposed on them in irregular patches and granules (fig. 3). In teased preparations these filaments are seen to largely end in a thick plexus between the columnar cells (fig. 3), but some of them also end in the cells which are developing or have developed a nematocyst (fig. 5). Not infrequently a filament may be seen having on its course a mass of granular protoplasm containing a nucleus (fig. 2). Traced downwards these filaments appear to be connected with a deeply-placed nerve network, 1 which in turn is in close relation to the muscle-fibres which are placed immediately upon the supporting lamella. If the ectoderm of the gonophore- bearing region of a specimen killed in the autumn be examined, it will be found to consist of externally a cuticle with columnar cells underlying it, and then an indefinite number of ganglion-cells and cells 1 Pig. 5 is an accurate drawing made with the aid of a camera lucida of a portion of this nerve complex, which by good fortune was isolated in a teased osmic acid preparation. It exhibits a remarkable fact in the arrangement of this primitive nervous system, namely, that some of the ganglioncells are enclosed in a fine reticulum, formed by the breaking up of filaments derived from the general nerve network. Since this drawing was made the remarkable researches of Golgi, Ramon y Cayal, Kolliker, and others have demonstrated the existence of similar structures in the central nervous system of the higher animals.
HISTOLOGY AND DEVELOPMENT OF MYBIOTHELA PHRYGIA. 511 concerned in the manufacture of nematocysts (fig. 4). In some sections scanty patches of small cells similar to those described above may be seen, but they are now the exception and not the rule. On the other hand, over considerable regions the ectoderm may be even more decrepit than that described above, for even the columnar cell layer may be imperfect, while the basal cells are represented largely by irregular spaces. On the other hand, the muscular layer and supporting lamella are proportionately better developed. Measured from the external surface of the cuticle to the external surface of the supporting lamella the ectoderm at this period is from 30 to 35 fi in thickness. We thus see that as the season advances and the reproductive period comes to a close the ectoderm of the gonophore-bearing region becomes more and more exhausted of those small cells which are such a characteristic feature of it in the spring, and I think there can be little doubt that their disappearance is connected with the active formation of gonophores during the summer months. So far I have spoken of them merely as the small cells of the proximal ectoderm. It is necessary to see whether these cells are all alike, or whether they may be divided into one or more sets differing in their function and connections. In tracing the process of formation of the nematocysts I have said that they first appear as a rounded hyaline mass embedded in the protoplasm of a cell which then lies in the deeper part of the ectoderm. The smallest cells containing these masses may be only 10p in diameter ; but though they do not differ markedly from the bulk of the small cells in point of size, they do differ in one very important particular, namely, in the fact that they are always connected by a delicate process with the nerve network (fig. 5). On the other hand, we have in addition to these small cells which are in connection with the nerve network, and so often contain some trace of a developing nematocyst, other still smaller rounded cells, which appear, even with the highest
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