THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 35 
becoming inclosed in a common capsular envelope, and by 
repeated division and consequent increase in number of 
cells becoming a new sponge. This is reproduction by 
“budding.” The “buds,” or small groups of cells which 
separate from the parent sponge, are called gemmules. 
Reproduction in the more complex way occurs as follows: 
Some of the free amoeboid cells of the mesoderm (the mid- 
dle one of the three layers of the body wall) become en- 
larged and spherical in form. These are the egg cells. 
Other mesodermic cells divide into many small cells, which 
are oval with a long, tapering, tail-like projection. These 
cells are active, being able to swim by the lashing of the 
tapering tail. These are the fertilizing cells. The two 
kinds of reproductive cells may be formed in one sponge; 
if so, they are formed at different times. Or one sponge 
may produce only egg cells, another only fertilizing or, 
as they are called, sperm cells. Conjugation takes place 
between a sperm cell and an egg cell. That is, one of the 
small active sperm cells finds one of the large, spherical, 
inactive cells and penetrates into the protoplasm of its 
body. The two cells fuse and form a single cell, which 
may be called the fertilized or impregnated egg. This fer- 
tilized egg, remaining in the body mass of the parent 
sponge, divides repeatedly, the new cells formed by this 
division remaining together. The young or embryo sponge 
finally escapes from the body of the parent sponge, and 
lives for a short time as an active free-swimming animal. 
Its body consists of an oval mass of cells, of which those on 
one side are provided with cilia or swimming hairs. The 
cells of the body continue to divide and to grow, and the 
body shape gradually changes. The young sponge finally 
becomes attached to some rock, the body assumes the typi- 
cal cylindrical shape, an aperture appears at the free end, 
and small perforations appear on the surface. The sponge 
becomes full grown. 
Those of us who do not live in the vicinity of the sea 
