The development of the various forms of SPICULES is a subject about which little is yet known. Most spicules of which the development has been traced originate in a single dermal cell. The triradiate and quadriradiate spicules of Homocoela (Clathrinidae), as Minchin[263] has most beautifully shown, form an exception. Three cells co-operate to form the triradiate; these three divide to give six before the growth of the spicule is complete. A quadriradiate is formed from a triradiate spicule by addition of the fourth ray, which, again, has a separate origin in an independent cell, in fact a porocyte. The triradiate spicules of the Sycettidae, on the other hand, originate in a single cell,[264] but the quadriradiate spicules are formed from these by the addition of a fourth ray in a manner similar to that which has just been described for Clathrinidae. [Illustration: FIG. 118.—Development of monaxon spicules. A, from _Spongilla lacustris_, showing the single scleroblast. (After Evans.) B, a very large monaxon, from _Leucosolenia_, on which many scleroblasts are at work. (After Maas.)] Monaxon spicules if not of large size undergo their entire development within a single scleroblast (Fig. 118, A). In some cases if their dimensions exceed certain limits, several cells take part in their completion; some of these are derived from the {233}division of the original scleroblast, others are drawn from the surrounding tissue. In _Tethya_, for example, and in _Leucosolenia_[265] the scleroblasts round the large monaxon spicules are so numerous as to have an almost epithelioid arrangement. The large oxeas of _Tetilla_, _Stelletta_, and _Geodia_, however, are formed each within a single scleroblast.[266] [Illustration: FIG. 119.—Development of spheraster. A, of _Tethya_, from union of two quadriradiate spicules. (After Maas.) B (_a-e_), of _Chondrilla_, from a spherical globule. (After Keller.)] Triaenes have been shown[267] to originate as monaxons with one swollen termination, from which later the cladi grow out. Information as to the scleroblasts in this case is needed. The value of a knowledge of the ontogeny of microscleres might be great. Maas believes that he has shown that the spherasters of _Tethya_ are formed by the union of minute tetractine calthrops (Fig. 119, A). If this view should be confirmed, it would afford a very strong argument for the Tetractinellid affinities of _Tethya_. [Illustration: FIG. 120.—Stages in the development of the microscleres of _Placospongia_. (After Keller.)] Keller,[268] on the other hand, finds that the spherasters of the Tetractinellid _Chondrilla_ {234}originate as spheres (Fig. 119, B); and spheres have been observed in the gemmule of a _Tethya_; no spherasters were as yet present in the gemmule, and spheres were absent in the adult.[269]