MATERIALS FOR A MONOGRAPH OF THE ASCONS. 531 



subject is ripe for further investigations, and that at present 

 no certain conclusion can be drawn as to the fate of the 

 mother-cell. 



Of the origin of the triaxon spicule of Hexactinellids we 

 know at present nothing whatever. But with regard to the 

 tetraxon siliceous spicule, the evidence is all in favour of an 

 origin within a single mother-cell. Schulze's observations on 

 Plakina (1880) are, so far as I know, the earliest in this field. 

 The author figures in the plainest manner the tetractiues in the 

 young sponge as enclosed each in a single cell (pi. xxii, fig. 29), 

 and the theory is advocated that the primitive siliceous spicules 

 were irregular thorny bodies, in which the rays first became 

 concentrated in a point, giving rise to irregular multiradiate 

 spicules ; that then the number and direction of the rays became 

 fixed, so that there were formed hexactines and tetractines ; and 

 that from these forms arose triactines, diactines, and even 

 monactines, by reduction of the rays (1880, p. 445). A year 

 later Schulze figured and described in Corticium "a dis- 

 tinctly formed regular tetractine " " in a rounded mesoderm 

 cell containing a few pigment granules," situated close to the 

 outer surface of the sponge ; the spicule was '' still so com- 

 pletely embedded in the protoplasm of the cell that its tips 

 did not even project into the surrounding ground substance " 

 (1881, pp. 426, 427, pi. xxii, fig. 10). 



The next author to deal with the origin of the tetractines 

 is Sollas (1888). He says, " In the Choristida all the spicules, 

 both large and small, originate each in a single scleroblast, 

 which persists throughout the life of the spicule. The sclero- 

 blast in the case of the large spicules is a large granular cell, 

 extending all round the spicule, which it has formed as a 

 siliceous secretion'^ (p. xlvi). Reference is then made to two 

 figures (pi. ii, fig. 20; pi. xiii, fig. 10), both of which, however, 

 only show one arm of the spicule, the rhabdome, with a cell 

 upon it; but in Tribrachion Schmidtii the author figures 

 very clearly entire orthodisenes of various sizes, each with a 

 single scleroblast upon the rhabdome (pi. xvii, figs. 12 and 

 20). On the other hand, Sollas also figures three cases in 



