GEMMATION OF MOLLUSCA AND VERTEBRA! A. 557 



body, but from a kind of stolon within it ; and they 

 differ from their parent-stock in having organs of attach- 

 ment to each other, whereby they hold together in long 



Fig. 299. AGGREGATE SALP^E. Fig. 300. SOLITARY SALPA. 



chains (fig. 299). These, in their turn, being furnished with 

 true generative organs, give origin to the solitary SalpaB (fig. 

 300) by ordinary sexual reproduction ; whilst the solitary 

 Salpa never comes to possess any sexual apparatus, but merely 

 continues the race by gemmation from its internal stolon. 



729. In the Vertebrated classes, as in the higher Mollusca, 

 we lose all trace of propagation by gemmation as an ordinary 

 method of multiplication. Yet there is evidence that this 

 power is not altogether extinguished, even in Man. For we 

 not unfrequently hear of " monstrosities by excess," that is, 

 of cases in which the body possesses a superfluity of some of 

 its parts ; the simplest cases being those of double thumbs or 

 of six fingers on each hand, and the gradation being so conti- 

 nuous from these to cases like that of the Siamese twins (in 

 which there are two complete bodies united only by a cross 

 band), as to make it evident that they are all referable to one 

 common principle. And although it has been commonly 

 believed that monsters with two heads and one body, or with 

 two bodies and one head, or with supernumerary legs or arms, 

 are results of the partial " fusion" of two distinct germs at an 

 early period, yet there is now far stronger reason to believe that 

 they proceed from a kind of attempt at multiplication by 

 fission or gemmation, that is sometimes made by a single germ 

 at a time when its grade of development corresponds with 

 that of a Hydra or Planaria ; which attempt, under peculiarly 

 favourable circumstances, may proceed to the full length of 

 production of two complete bodies. (See 390.) 



Sexual Reproduction, or Generation. 



730. We now have to notice the most important features 

 of the proper Generative process, which differs from the 



