24 
specialized and independent portion (vitellogene gland) of the female glands; instead 
of developing into embryos, the yolk cells form a follicle-like covering for the true 
germ cell and play an important role in the nutrition of the latter as it undergoes 
further development; (3) a shell surromidihg 
the germ cell and the vitelline cells, and pro- 
vided at one end with a cap or operculum. 
The eggs escape from the uterus of the adult 
through the vulva, are carried to the intestine 
of the host with the bile, then pass through the 
intestines with the contents of the latter, and 
are expelled from the host with the fecal mat- 
ter. Many of them become dried and then 
undergo no further development, but others are 
naturally dropped in the water in marshes, or, 
being dropped on dry ground, they are washed 
into the water by the rain, or are carried to a 
more favorable position by the feet of animals 
pasturing in or passing through the fields. After 
a longer or shorter period of incubation, which 
varies with the temperature, a ciliated embryo 
Fig. 21. — Egg of the Common Liver Fluke 
{Fasciola hepatica) examined shortly after 
it was taken from the liver of a sheep; at 
one end is seen the lid or operculum, o; 
near it is the segmenting ovum, e; the 
rest of the space is occupied by yolk cells 
which serve as food; all are granular, 
but only three are thus drawn, x 680. 
(After Thomas, 1883, p. 281, fig. 1.) 
-f 
...-k 
(miracidium) is developed. At a 
temperature of 20° to 26° C. the 
miracidium may be formed in 10 
days to 3 weeks; at a temperature 
of 16° C. the development takes 2 
to 3 months; at 38° C. it ceases 
entirely. Experiments have shown 
that as long as these eggs remain 
in the dark the miracidium will not 
escape from the eggshell; accord- 
ingly it will not escape during the 
night. When exposed to the light, 
however, or when suddenly brought into contact with cold water, the 
bursts the cap from the eggshell, crawls through the opening and becomes a — 
Fig. 22. — Egg of tbe Common Liver Fluke containing a 
ciliated embryo (miracidium) ready to hatch out: d. 
remains of food; e, cushion of jelly-like substance: /, 
boring papilla; Ti, eye-spots: germinal cells, x 680. 
(After Thomas, 1883, p. 283, fig. 2.i 
