MATERIALS FOE A MONOGRAPH OF THE ASCONS. 539 



this author's habitual method of stating dogmatically, and as 

 if from his own observation, propositions whieh are the result 

 merely of imagination, and deduced a priori from, in this 

 instance at least, unsound premises. 



In the present decade Dendy has put forward views on the 

 origin of calcareous spicules, which can best be made clear by 

 quotations from his works. "It is generally admitted," he 

 says, " that both calcareous and siliceous spicules originate 

 within special mother-cells, but probably in both cases they 

 subsequently receive additional layers of the calcareous or 

 siliceous material from other cells. . , . There are probably 

 two kinds of calcoblasts, primary and secondary. The primary 

 calcoblasts are the mother-cells in which the spicules originate, 

 and the secondary calcoblasts are the cells which secrete 

 additional layers of calcareous matter around the spicule after 

 it has been formed" (1891 [1], p. 25 ; compare 1891 [2], p. 15). 

 He states his belief in the same passage that the calcoblast is 

 " an ordinary stellate mesodermal cell." " Tlie spicule sheaths 

 .... are formed by a slight concentration of the structure- 

 less mesodermal jelly around the spicule" (1893, p. 223). 

 Supposed primary calcoblasts are figured on the uniaxial 

 spicules of Leucandra phiUipensis, and it is stated that 

 " their characters certainly justify the assumption that they 

 are but slight modifications of ordinary stellate cells" (1893, 

 p. 225, figs. 44 — 47). Dendy quotes my former statements 

 (1892 [1], p. 265, fig. 15, a, b), which I have stated above to 

 be erroneous, as to the presence of a fourth cell at the centre, 

 in addition to the three on the rays, of the triradiates of CI. 

 coriacea ; and considers that the cells on the rays represent 

 the secondary calcoblasts, '' for we can hardly suppose that 

 the spicule is originally formed by more than one cell" (1893, 

 p. 225). In so far as this remark applies to the triradiate 

 systems, I think the observations described above sufficiently 

 refute it, but in so far as Dendy wishes to express his conception 

 of what constitutes a true spicule in general, as distinguished 

 from compound spicular systems, I quite agree with him. 



In considering Deudy's views of spicule formation, it must 



