GEMMULATION 



231 



and are finally freed, like the buds of the Hexactinellid Zopho- 



calyx (Fig. 113). 



The comparison of the process of development on the one hand 



by gemmules, and on the other by larval development, is of some 



interest.^ In both cases two cell 



layers — a dermal and a gastral — are 



established before the young sponge 



has reached a functional state. Dif- 

 ferences of detail in the formation of 



the chambers occur in the gemmule ; 



these find parallels in the diiTerences 



in the same process exhibited by the 



larvae of various groups of sponges. 



On the other hand, the order of tissue 



differentiation is not the same in the 



gemmule as in the larva. 



Of the reproduction of Tetrac- 



tinellida extremely little is known. 



Spermatozoa occur in the tissues 



in profusion and are doubtless 



functional, but larvae have been 



seldom observed. 



In Hexactinellida the place of 

 sexually produced larvae is taken by 

 bodies of similar origin to gemmules 

 but with the appearance of paren- 

 chymulae. Ijima has indeed seen a 

 few egg-cells in Hexactinellids.^ He 

 finds, however, that archaeocyte con- 

 geries occur in abundance, and there is 

 good reason to believe with him that 

 these are responsible for the numerous 

 parenchymula-like asexually pro- 

 duced larvae he has observed. The 



'"devet-;„erof1hT\ri™diat: discovery of " asexual larvae" was 

 spicules of Sycon setoswm. x gj.g(; jQade by Wilson in the Monaxonid 

 1200. (After Maas.) ^sperdla ; in this case the asexual 



larva is, as far as can be detected, identical with that developed 



from the fertilised egg. A similar phenomenon, the production 

 1 Maas, loc. cit. p. 284. '' J- Coll. Japan, xv. 1901, p. 180. 



Fig. 116. — Development of the tri- 

 radiate and quadriradiate spicules 

 of Olathrina. (1) Three solero- 

 blasts ; (2) each has divided : the 

 spicule is seen in their midst ; (3) 

 addition of the fourth ray by a 

 porocyte. p, Dermal aperture of 

 pore ; r, fourth ray. (After 

 Minchin.) 



