14 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, Tcenia 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 stomiach, 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 t^pe-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. 



