44 THE DOCTRINE OF DESCENT. 
If the egg of a tape-worm, by chance and good luck, 
strays into a congenial stomach,—for example, the egg 
of the human tape-worm, Teenia solium, into the stomach 
of a pig, the embryo wanders out of the stomach in 
which it quitted the egg, and makes its way into the 
muscles, where it swells out into a sort of cyst. This 
cyst is the first intermediate generation. It produces a 
peg-shaped gemmule, which, however, fails of its object 
as long as the “bladder worm,” or “Gargol,” remains in 
the flesh of the pig. It is only when this comes, raw or 
imperfectly cooked, into the human stomach, that the 
time has arrived for the release of the pupa. It emerges 
from its parent the cyst, and the pupa, in which we now 
recognize the head and thorax of the tape-worm imago, 
represents a second intermediate generation. Its pro- 
ductiveness is forthwith displayed; it becomes elon- 
gated, and as its ribbon-like form increases, shooting 
out from the posterior portion of the cervix, the more 
distinctly marked become the transverse stripes and 
“somites ;” in other words, the individuals of the third 
or sexual generation. 
In the evolutionary cycles just discussed, there is an 
alternation of asexual and sexual reproduction; and 
before examining some other cases of asexual multi- 
plication, we must make ourselves acquainted with the 
facts of sexual reproduction. 
The characteristic of this is, that it requires for the 
generation of the new individual the union of two 
different products or morphological elements, the ovum 
and the sperm. The ovum is always, in the first in- 
stance, a simple cell, of which the nucleus is termed the 
germinal vesicle, and the nucleole the germinal spot. 
