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 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]]
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.
Fig. 120.—Stages in the development of the microscleres of Placospongia. (After Keller.)