504 E. A. MINCHTN. 



then to proceed to describe the origin and formation of the 

 gastral rays. 



(a) The Development of the Triradiate Systems. — 

 The origin of the triradiate systems can be traced back to 

 certain cells of the dermal Hat epithelium, which have wan- 

 dered inwards and have come to lie between the flat epithelium 

 and the collared epithelium of the gastral layer. This is true 

 equally of the first spicules formed in the young sponge 

 after the fixation and metamorphosis of the larva, and of all 

 spicules formed during subsequent growth in the adult sponge. 

 The immigration of the spicule-secreting cells in the young 

 fixed embryos first causes the dermal layer to become difieren- 

 tiated into a more internal connective-tissue layer and an 

 external contractile epithelium. In the adults, where the 

 connective-tissue layer is well established, the skeletogenous 

 cells migrate from the epithelium into it, so that the connec- 

 tive-tissue layer is continually being recruited, as it were, from 

 the dermal epithelium. 



In the present memoir I propose to deal more particularly 

 with the formation of the spicules in the adult, a few stages 

 from embryos being described for comparison. I have observed 

 the formation of the triradiate systems in the adults of Clath- 

 rina coriacea, CI. reticulum, CI. cerebrum, CI. con- 

 torta, and CI., sp. dub., and in the embryos of CI. cerebrum, 

 CI. reticulum, and Ascandra falcata. Since in all these 

 cases the development proceeds in an essentially similar 

 manner, and difi'ers only in points of detail, I have taken 

 coriacea as a type (figs. 1 — 16), in order to avoid unnecessary 

 multiplication of similar figures. The description to follow 

 refers, therefore, to coriacea, unless the contrary is stated. 

 The differences shown by other types will be mentioned in 

 their place, and where necessary illustrated by figures. 



The mother-cells of the spicules can be found without diffi- 

 culty in actively growing parts of the sponge (figs. 1 — 3 

 and 10, PI. 38, act. bl.). They are compact, irregularly rounded 

 cells, which in all the characters of their nucleus and cyto- 

 plasm show an agreement amounting to identity with the 



