33 



dra is developed from an impregnated ovum, and propa- 

 gates by gemmation an individual which in its turn deve- 

 lopes fertile ova. Not that the processes always do alter- 

 nate with this regularity, but they may do so. 



It is obvious that by calling them e alternate generation/ 

 we should not explain their nature any more than if we 

 were to say that the polype from the bud, or the monad 

 from the fission, was " spontaneously or equivocally gene- 

 rated independently of any impregnation/' or that they 

 resulted " from the individualization of a previously orga- 

 nized tissue." All these would be but so many different 

 modes of expressing the fact, with the addition of demon- 

 strable misconception of its nature by the advocates of the 

 equivocal generation hypothesis. 



Let us further suppose the hydra from the bud to as- 

 sume a different form from the egg-born hydra out of which 

 it proceeded, and to be oviparous ; but the ova to produce 

 only the lower form of hydra, which is gemmiparous ; a 

 lower and higher form alternately appearing, the one from 

 an egg, the other from a bud. These phenomena might 

 likewise be defined to be ' an alternating generation' ; but 



1838, p. 298; reproduced by Siebold, Lehrbuch der Vergl. Ana . 

 p. 23.) Such an objection would have force only were a shell or 

 chorion the essential part of an ovum. But this essentiality being the 

 impregnated germ-cell, which alone represents the ovum in the Poly- 

 gastria, it may be developed into the embryo, without the phenomenon 

 of exclusion, as in the case of the monadiform embryo of the Medusa, 

 and as Steenstrup describes to be the development of the gregariniform 

 larvae of the Distoma tarda. This is the course of development which 

 might be anticipated in the representative of an ovum in a micro- 

 scopic monad which itself represents an animated cell or ovum. The 

 relation however of those minutest forms of Polygastria, e. g. Chlamy- 

 domonas pulvisculus, to the larger and more complex forms has still to 

 be determined. If they should prove to be embryonic forms, the sig- 

 nificance of their close repetition of the first stages in the formation of 

 a germ-mass in the ova of higher animals by undergoing their spon- 

 taneous fission within a common covering would be intelligible. Com- 

 pare fig. 14, p. 24, and figs. 33-44, p. 77, of my " Lectures on the 

 Invertebrate Animals," 8vo, Longmans, 1843. 



C 



