MATERIALS FOR A MONOGRAPH OF THE ASCONS. 517 



of development common to all, with a number of constant and 

 definite stages, as follows : 



(1) Formation of "trios" by immigration of cells from the 

 flattened epithelium. 



(2) From the trios arise the '^ sextets " by division of each 

 cell into two. 



(3) The spicule appears with each of its rays corresponding 

 to two sister-cells of the sextet, i. e. to two cells which have 

 arisen from the division of one of the cells of the trio. 



(4) As the rays increase in length the inner formative cells 

 of the sextet remain at the apices, the outer formative cell at 

 the bases, of the rays. 



(5) Disappearance of the apical formative cells. 



(6) The basal formative cells, after building up the rays to 

 their full thickness at their bases, migrate slowly to the 

 extreme tips of the rays, where they remain adherent as the 

 definitive spicule-cell. 



{b) The Origin of the Fourth or Gastral Ray of the 

 Quadri radiate Spicules. — It has already been stated that 

 the three basal rays of the quadriradiate spicule develop 

 exactly as do the triradiate spicules, and that the fourth ray is 

 an adventitious structure, derived from an entirely different 

 source. In fact, while the mother-cells of the triradiate 

 system are derived from the fiat epithelium, that is to say, 

 from the dermal surface of the body-wall, the mother-cell of 

 the fourth ray is derived from the gastral surface of the body- 

 wall, and from the layer of cells which we have termed above 

 the porocytes. 



Referring for a moment to what has already been stated 

 with reference to the origin of the porocytes, we have seen 

 that they originate in two ways in two different regions : (1) 

 at the blind ends of diverticula, as well as throughout the 

 sponge generally, from large granular cells of the flat epithe- 

 lium which migrate inwards through the dermal layer, and 

 come to lie between the collar-cells ; (2) at the tip of the 

 osculum, from the layer of large granular epithelial cells 

 lining the interior of the oscular rim, which in their turn are 



VOL. 40, PAKT 4. — NEW SEE. O O 



