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 Désé (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 
