154 INTERPOLATION OF A CONTINUOUS 



Hepaticse, among Cryptogamic plants the spores produced 

 by the protomorphic gemma which becomes the capsule, 

 give origin directly to the typical frond, but in mosses we 

 have a second form interposed viz., the Protonema. 



In the orthomorphic gemmation the number of links is 

 still more variable, reaching perhaps their maximum among 

 animals in the ten or twelve generations of larval Aphides, 

 but in plants running on to an indefinite extent, and pro- 

 bably depending much on external circumstances, while 

 the remarks which have been already made on the pro- 

 liferous Medusae show that pullulation, though less common, 

 is not less variable in the gamomorphic stage. 



Various other facts might be adduced to show that the 

 processes do not stand at all on the same footing. Thus 

 we find that even in species in which the sexual gemmae are 

 detached as independent organisms, it is comparatively 

 rare for the non-sexual shoots, which have pullulated from 

 the original typical form, to be detached in the same way.* 



We may observe farther, there are numerous cases in 



there is a succession of cystic forms before the characteristic heads of the 

 Echinococcus are produced. In this view the Scolex of the Cestoid worm 

 the Tsenia-head is represented, not by any Trematode Eedia, primary 

 or secondary, but by the early condition of the Distoma itself, including 

 the cercarian phase through which it passes in the usual course of its de- 

 velopment (but not, as it would seem, universally in all the species), and 

 the proglottides of the Tapeworm have no farther representations among 

 the Trematoda than what may be furnished by the reproductive organs 

 developed at a later period in the fully formed Distoma. 



* The exceptions are unimportant, and occur principally in the aber- 

 rant cases before noticed among the Articulata, or, again, among phanero- 

 gamic plants, as in the deciduous bulbs of Begonia, Allium, and various 

 Lilies, Marchantia, and some other Hepaticce, &c. In a few plants such 

 deciduous buds may occasionally take the place of seeds, especially if the 

 maturation and impregnation of the ovules is prevented by force of cir- 

 cumstances. An inflorescence bearing such bulbs is termed by botanists 

 viviparous, though in the received signification of the word it is here mis- 

 applied, the only truly viviparous plants being such as the Mangrove, in 

 which the embryo germinates while still attached to the plant. 



