FIG. 57. Three Stages in the Development of the Gemmule of a Fresh- 

 water Sponge, Ephidatia fluviatilis, Johnston. After Zykoff. (Only 

 one-half the gemmule is shown in each case.) 



A. g. Gemmule cells, destined to give rise to the young sponge, p. 



Amoeboid cells, g.e. Glandular cells, which secrete the inner horny 

 (chitinous) layer, i.e. a. Amphidisc spicules, shown at an earlier 

 stage of development by a', o. Needle-shaped spicules which form 

 the skeleton of the adult sponge. 



B. Second stage. The inner coating of chitin (i.e.) is completed, and 



the amphidisc spicules (a) placed in position. 



C. Final Stage. The glandular cells (g.e. ) have now retreated to the 



exterior, leaving the completed layer of amphidisc spicules behind. 

 Magnified considerably. 



