DEVELOPMENT OF SPICULES 



division of the original scleroblast, others are drawn from the 

 surrounding tissue. In Tethya, for example, and in Leucosolenia 1 

 the scleroblasts round the large monaxon spicules are so numerous 

 as to have an almost epithelioid arrangement. 



The large oxeas of Tetilla, Stelletta, and Geodia, however, are 

 formed each within a single scleroblast. 2 



i 



a 



E 



Fig. 119. Development of spheraster. A, of Tethya, from union of two quadri radiate 

 spicules. (After Maas.) B (a-e), of Chondrilla, from a spherical globule. (After Keller.) 



Triaenes have been shown 3 to originate as monaxons with 

 one swollen termination, from which later the cladi grow out. 

 Information as to the scleroblasts in this case is needed. 



The value of a knowledge of the ontogeny of microscleres 

 might be great. Maas believes that he has shown that the 

 spherasters of Tethya are 



formed by the union of 

 minute tetractine cal- ^ 

 throps (Fig. 1 1 9, A). If %f 

 this view should be con- ^ v 

 firmed, it would afford a 

 very strong argument 

 for the Tetractinellid 

 affinities of Tethya. 



Keller, 4 on the other 

 hand, finds that the spherasters of the Tetractinellid Chondrilla 





a 



Fig. 120. Stages in the development of the micro- 

 scleres of Placospongia. (After Keller.) 



1 Maas, SB. Ale. Miinchcn, xxx. 1900, p. 553, and Zcitschr. wiss. Zool. lxx. 

 1901, p. 265 ; see also Sollas, Ann. Mag. Nat. Hist. (5) ix. 1880, p. 401. 



2 Sollas, Challenger Monograph, xxv. 1888, p. xlv. 



3 Sollas, ibid. pp. 13 and 34, pi. v. 4 Zeitschr. wiss. Zool. lii. 1891, p. 294. 



