The comparison of the process of development on the one hand by gemmules, and on the other by larval development, is of some interest.[260] In both cases two cell layers—a dermal and a gastral—are established before the young sponge has reached a functional state. Differences of detail in the formation of the chambers occur in the gemmule; these find parallels in the differences in the same process exhibited by the larvae of various groups of sponges. On the other hand, the order of tissue differentiation is not the same in the gemmule as in the larva. [Illustration: FIG. 116.—Development of the triradiate and quadriradiate spicules of _Clathrina_. (1) Three scleroblasts; (2) each has divided: the spicule is seen in their midst; (3) addition of the fourth ray by a porocyte. _p_, Dermal aperture of pore; _r_, fourth ray. (After Minchin.)] Of the reproduction of Tetractinellida extremely little is known. Spermatozoa occur in the tissues in profusion and are doubtless functional, but larvae have been seldom observed. [Illustration: FIG. 117.—Three stages in the development of the triradiate spicules of _Sycon setosum_. × 1200. (After Maas.)] In Hexactinellida the place of sexually produced larvae is taken by bodies of similar origin to gemmules but with the appearance of parenchymulae. Ijima has indeed seen a few egg-cells in Hexactinellids.[261] He finds, however, that archaeocyte congeries occur in abundance, and there is good reason to believe with him that these are responsible for the numerous parenchymula-like ASEXUALLY PRODUCED LARVAE he has observed. The discovery of "asexual larvae" was first made by Wilson in the Monaxonid _Esperella_; in this case the asexual larva is, as far as can be detected, identical with that developed from the fertilised egg. A similar phenomenon, the production {232}of apparently identical larvae by both sexual and asexual methods, has been observed in the Coelenterate _Gonionema murbachii_.[262] Artificially, sponges may be reproduced with great advantage to commerce by means of cuttings. Cuttings of the bath sponge are fit to gather after a seven years' growth.