70 GERM-CELL CYCLE IN ANIMALS 



species the buds become free (discontinuous bud- 

 ding). 



Gemmules are groups of cells (statocytes) which 

 occur at certain times of the year in the bodies of 

 fresh- water sponges and in many marine species. 

 These gemmules acquire a resistant covering and 

 serve to preserve the race during the winter in the 

 north or the dry season in the south. The peculiar 

 " budding" observed in Tethya by Deso (1879, 1880) 

 may be a sort of gemmule formation (see p. 76). 



The eggs and spermatozoa are situated in the 

 middle layer (so-called mesoderm) and in most 

 cases seem to become ripe at different times in the 

 same sponge. Fertilization is apparently similar 

 to this process in other METAZOA. The fertilized 

 ovum is holoblastic ; the free-swimming ciliated 

 larva becomes fixed, and then metamorphoses into 

 a young sponge. 



The body wall of the sponge consists of two distinct 

 layers, an outer dermal layer and an inner gastral 

 layer, and an intermediate jelly-like stratum con- 

 taining ameboid wandering cells. The various sorts 

 of cells in these layers are indicated in the table on 

 page 71 (from Minchin, 1900, p. 62). 



The reproductive cells lie in the jelly-like middle 

 layer, but all of the cells in this layer are not repro- 

 ductive. 



The origin of the archeocytes from which the re- 

 productive cells arise can easily be pointed out in 

 the comparatively simple development of Clathrina 

 blanca (Minchin, 1900). In this species a ciliated 



