b ENTOZOA FOUND IN MAN. 



fact, if we compare with one another the embryo, the 

 head, and the rings of a tape-worm, it may be readily 



happen that the individual which is produced does not resemble 

 the individual which is the producer. It is known that in the 

 case of batrachian animals, and of insects, the larva which issues 

 from the ovum does not resemble the parent, but that sooner or 

 later it acquires the shape and organisation of the parent by 

 metamorphosis. In the case of some other animals, the indi- 

 vidual which issues from the ovum, difiFering also in form and 

 organisation from the individual which produced it, does not 

 become metamorphosed into an individual resembling the parent, 

 but perishes without ever reaching the adult state ; and there are 

 other individuals to which it gives birth, by means of germs, which 

 acquire the form of the original parent, and which, in their turn, 

 produce ova. The individual which has issued from the ovum, 

 does not resemble, either in form, or in organisation, that which 

 produced it, nor does it in any greater degree resemble its 

 progeny; this latter possesses the form of the first parent, or it 

 acquires it by a metamorphosis. There are consequently two 

 very distinct phases of generation ; but sometimes this second 

 generation does not arrive at the adult condition, but reproduces 

 a third, differing from itself and from that which preceded it, and 

 this third generation alone assumes the type of the primitive 

 parent. 



By the term Alternate Generation, or Digenesis, is therefore 

 understood the succession of dissimilar generations, sexual and 

 non-sexual, after which the primitive type is resumed. 



It frequently occurs that an individual belonging to one of 

 these phases of generation (ordinarily that in which genital organs 

 are not possessed) produces new individuals similar to itself, and 

 these, in their turn, give birth to other individuals similar to 

 themselves before either of them produces individuals of a dis- 

 similar character. These similar individuals, born of a common 

 stock and successively one from the other, cannot be considered 

 as constituting new phases of generation, for they do not form a 

 more advanced stage in the evolution of the animal which they 

 represent, and they only multiply the individual-stock ; the dis- 

 similar individuals, on the contrary, always form a stage in 

 advance towards the adult state. The larva which produces a 

 succession of ten or twelve individuals, born one from the other 

 by gemmation, and similar to each other, has not definitively ten or 



