220 PHYLUM ANNELIDA. 
When the eggs of an earth- 
worm are liberated, they are 
surrounded by a sheath of 
gelatinous stuff, believed to 
be secreted by the saddle. 
As this is peeled off towards 
the head, spermatophores are 
also enclosed. 
Development. — Many 
cocoons are made about the same 
time, and each contains numerous 
ova, and also packets of sperms, 
so that fertilisation takes place 
outside the body. These cocoons 
are. buried in the earth a few 
inches below the surface. They 
measure about a quarter of an inch 
in length. 
The favourite time for egg- 
laying is during the spring and 
summer, though it may be con- 
tinued throughout the whole year. 
The earthworm of the dungheap 
(Z. fetidus) makes this a habit, 
induced probably by the warmth 
of its environment. 
Of the many ova in the cocoon 
of ZL. derrestris, only one comes 
to maturity, while in Z. fetidus a 
few, and in Z. communzs two may 
do so. But in the last species 
Fic. 114.—Stages in the develop- 
ment of earthworm. — After 
Wilson. 
1. Two-celled stage; 4.c., polar bodies. 
2. Blastula; JZ, a primary mesoblast. 
3. Gastrula stage; Zc., ectoderm or 
epiblast ; £7., endoderm or hypo- 
blast,-in process of being covered 
by the small ectoderm cells. Note 
the widely open blastopore; JZ, 
mesoblast cells. 
4. Longitudinal section in late gastrula 
stage, showing germ-bands; ec, 
ectoderm; ex., endoderm; JZ, 
mouth; sz, stomodeum; wm, 
primary mesoblasts; Vé., neuro- 
blasts; #c., nerve-cord; JV., ne- 
phridioblasts ; ms., mesoderm 
bands; xpc., incipient nephridia. 
