MATERIALS FOR A MONOGRAPH OF THE ASCONS. 509 



rays up to 5 |u in length (figs. 5 and 48)^ sometimes even in 

 much larger spicules (fig. 23). But in spicules with rays more 

 than about 5 ju in length complete fusion has taken place, as 

 a rule, between the rays (fig. G), and instead of three separate 

 spicules^ we have a single spicular system. 



The spicule continues to grow, and at the same time its 

 formative cells begin to shift their position. Each ray, as we 

 have seen, is surrounded by two formative cells, one lying 

 internally to it, the other externally. At first the ray is small 

 in proportion to its formative cells, and completely embedded 

 in them ; but it soon grows to a length far exceeding the 

 diameter of the cells. As it does so, the inner formative cell 

 remains at the apex of the ray, impaled, as it were, upon its 

 point, while the outer formative cell remains at the base of the 

 ray (PI. 38, fig. 7). In consequence of these relations, the six 

 cells of the sextet become widely separated from one another, 

 at least as far as their principal mass is concerned. The exact 

 limits of the cells are very difficult to determine on account of 

 the refraction of the spicule. It is evident from the manner 

 in which the spicule grows that it must be covered everywhere 

 by a layer of protoplasm. But except in the vicinity of the 

 nucleus the investing layer of protoplasm cannot, as a rule, be 

 made out, so that the bodies of the formative cells appear dis- 

 tinct from one another and adherent to the rapidly growing 

 spicule. The spicule does not appear naked when exposed by 

 the separation of its formative cells, but it is covered every- 

 where by a delicate layer or sheath, very hard to see in freshly 

 made preparations, but very distinct in preparations kept some 

 time in glycerine, in which the spicule is gradually dissolved. 

 This is the first appearance of the well-known spicule sheath. 

 It becomes more distinct as the spicule grows in size, but can 

 scarcely be distinguished until the spicule ray has outgrown its 

 formative cells, and it appears as a structureless membrane 

 or layer intervening between the spicule and the secreting 

 protoplasm. 



The result of these changes is a stage very commonly found 

 in the growing spicule, and very characteristic. PI. 38, fig. 8, 



