71 



germinal vesicle, of its share of that seminal virtue which 

 was incorporated with the parent-cell in the act of impreg- 

 nation and distributed by its successive divisions. 



The course of development by which the ovum of the 

 Distoma has been converted into the Gregariniform larva 

 is as simple and as short as could be conceived, in order to 

 support the idea of such a conversion. A free moving 

 animalcular entozoon is, however, the result : a parasite 

 closely resembling those of the genus Gregarina as defined 

 by Kolliker and Siebold. This early larval form of the 

 Distoma is explained to be a wet-nurse (Amme), or rather 

 a grand-nurse (Grosse-amme), in the figurative language of 

 Steenstrup. What it truly is, and what its powers are, and 

 on what they depend, can only be understood by a recog- 

 nition of the essential nature and signification of the first 

 steps in its development, viz. the diffusion of the spermatic 

 force in the formation of the germ-mass. 



Now we might expect, from the analogy of higher ani- 

 mals, that certain of the constituent corpuscles of this 

 germ-mass would perish as such in order to combine and 

 form the tissues and organs of the Distoma. But the fer- 

 tility of Nature's combinations and variations is inexhaust- 

 ible ! Instead of that ordinary process, several of the germ- 

 corpuscles of the Gregariniform larva set up independently 

 the centralizing actions of assimilation and spontaneous 

 fission, and so constitute as many secondary germ-masses, 

 in each of which a greater proportion of the constituent 

 corpuscles coalesce and combine to form the tissues of an 

 animal, like a Cercaria, than happened in the development 

 of the Gregariniform parent. Under the form of Cercariae 

 these secondary larvae, as they may be termed, burst the 

 integument of the primary one and are set free ; yielding 

 one of the most striking examples of the multiparous cha- 

 racter of the Trematode ovum ; the multiplication of the 

 larval forms which precede the ultimate oviparous indivi- 

 dual being simultaneous, not consecutive as in the Aphis. 



F 2 



