536 E. A. MINCHIN. 



He believed the external protoplasm of the cells of the dermal 

 layer to be " completely fused and modified into a homogeneous 

 and possibly contractile ground substance" (p. 252), and he 

 agreed with Haeckel in regarding the spicule sheath as a con- 

 densed layer of the ground substance closely surrounding the 

 spicule (p. 254). That Schulze at this period agreed with 

 Haeckel as to the mode of formation of the spicules may be 

 further inferred from the fact that in his work on the develop- 

 ment of Sycandra raphanus (1878), while figuring clearly 

 the monaxon spicules each in its secreting cell (pi. xix, figs. 

 10, 11), he yet stated in the text that the spicules arise in the 

 hyaline substance between the two layers. 



With the year 1879 we come to the only important obser- 

 vations, as apart from theories, on the formation of calca- 

 reous spicules which have been made since the sixties, namely, 

 those of Metschnikofi^. This author first demonstrated that 

 calcareous, like siliceous spicules, arise within cells, and not 

 free in the ground substance of the sponge body. Valuable 

 as were Metschnikoff's results, they are nevertheless not free 

 from important errors of misinterpretation as regards the 

 details of the development, though based on accurate and 

 careful observation. 



The specimens of Ascons which MetschnikofF investigated 

 seem to have been for the most part more or less contracted, 

 since in sections he found a dermal epithelium composed of 

 flask-shaped cells. We have already noticed his observations 

 on the porocytes, which he believed to be granular mesoderm 

 cells. In Clathrina primordialis he found these cells 

 giving rise to the skeleton. " The smallest calcareous spicules 

 are formed in the interior of such cells " (p. 361, and pi. xxii, 

 figs. 4, 5, 8, and 12 s.). This is a most important statement in 

 view of what has been said above, namely, that both the poro- 

 cytes and the spicule-forming cells originate from the dermal 

 epithelium. The cell masses surrounding the smallest spicules 

 do in fact resemble closely in appearance the contracted poro- 

 cytes, a resemblance due to the possession of similar cytological 

 characters derived from a common origin. Hence it is not 



