Page:Proceedings of the Royal Society of London Vol 60.djvu/56

48 The two component layers very soon begin to undergo changes of form and structure, which are best described separately, since the two layers develop more or less independently of one another, and a given stage in the development of one layer is not always found combined w ith one and the same stage in the development of the other. The dermal layer becomes divided (fig. 5) into two kinds of cells: (a) cells which retain the original form and characters and rem ain on the surface, and (b)cells w ith smaller nuclei, w hich sink below the outer epithelium and form a scattered layer between it and the

Fig. 5.—Section of stage about twenty-four hours after fixation. The left side is represented as slightly in advance of the right side. gastral cells. The form er (a)secrete each a single monaxon spicule, which appears first on the inner side of the nucleus, but soon grows out and projects free from the surface. The latter unite into groups and secrete the triradiate spicules. The monaxons appear first, as in Sycandra raphanus,* and begin to appear about twenty-four hours after fixation, the triradiates about twelve hours later. The dermal layer has thus become divided into two parts, which gradually assume the adult characters. I have not observed the origin of the pores.

The gastral layer, at first a com pact mass w ith no definite arrangement, soon begins to form a cavity (fig. 5). The cells assume a radiate arrangem ent, and a split-like lumen appears in the centre. Sometimes two or more such lacunar spaces arise, at first quite independent of one another, but later fusing to form a single gastral cavity, which soon becomes very large, causing the larva to increase considerably in size as a whole. A t first the cavity is surrounded on all sides by gastral cells, but as it increases in size a spot appears where gastral cells are wanting, and the cavity is limited only by dermal cells (fig. 6). This is the region of the future osculum, and the dermal cells at this spot form the future oscular rim, where|collar
 * Metschnikoff, loc. cit.