66 



SPONGES 



Flo. 56. 



Three stages in the development of a gemmule in Spongilla (after Zykoff). In A the amoebo- 

 cytes (statocytes), packed with refringent granules, are becoming aggregated at one spot, and 

 the parenchymal cells round them are taking an epithelial form and secreting an adventitious, 

 chitinous envelope ; still further away, the amphidiscs are being formed in scattered cells of the 

 parenchyma. In B the statocytes are densely packed and enclosed by the chitinous coat with 

 its secreting epithelium ; the amphidiscs are now passing between the cells of the latter. In 

 C the amphidiscs form a definitely arranged coat internal to the secreting epithelium, which is 

 now placed on the exterior, and is secreting a second chitinous envelope external to the amphidiscs. 

 amph, amphidiscs ; amph', a young amphidisc ; i.ch.e, internal, and e.ch.e, external, chitinous 

 envelopes ; gl.ep, glandular epithelium ; ox, oxeote spicules of the sponge ; par.c, parenchymal 

 cells ; st.c, statocytes or gemmule cells. 



