OTHER FORMS OF ALTERNATION. 169 



except in a few individuals, the majority remaining neuter 

 in function, if not in all cases absolutely devoid of the 

 structural characters of sex. 



Even, however, when such organs are normally matured 

 in all the individuals of the species, and when, as is ge- 

 nerally the case, traces of them do make their first appear- 

 ance along with the neighbouring viscera,* it is still a con- 

 stant rule that they lie in a latent condition long after the 

 full development of other parts, and are the last to take on 

 a state of functional activity. This only occurs on the for- 

 mation in their substance, at the proper period, of numerous 

 cells containing the peculiar reproductive corpuscules (sper- 

 matozoa and ova) ; and it is not till the development of 

 these, that the bodies in question, which up to this time 

 have been mere masses of parenchyma, can rightly be 

 called organs of reproduction. Hence probably the dif- 

 ference between the common case, and that of the species in 

 which these organs appear to be formed pro re nata, may 

 not amount to more than this, that in the former the gem- 

 mation extends only to the cells producing reproductive 

 corpuscules, while in the latter it embraces also the general 

 envelope or viscus in which they are contained. Or per- 

 haps in propriety we ought to make such a distinction be- 

 tween these formative cells and the stroma in which they 

 lie, as to regard the latter as merely a proliferous tract of the 

 parent's body like the base of the Teenia-head, or the cap- 

 sule of the Sertularian polypidom, or like the " funiculus" 

 of the Polyzoon presently to be noticed and to consider 



* This, however, is probably conceding too much, for it is now well 

 ascertained that the generative glands are not part of the organization as 

 originally laid down, at least in Mammalia and Birds, but that the large 

 masses occupying each side of the abdomen of the embryo at an early 

 stage of development, which are known as the Wolffian bodies, and have 

 themselves but a provisional part to fulfil, form the primordial matrices 

 upon which both the urinary and genital organs are developed. See a 

 farther reference to this point in the concluding Chapter. 



I 



