4o6 PHYSIOLOGY OF THE INVERTEBRAIA 



described by Drs. Dallinger and Drysdale * as follows — 

 " development from a germ or sporule of extreme minuteness, 

 and ou the attainment of maturity multiplication by fission, f 

 constantly and for an indefinite time ; but the vital power is 

 at intervals renewed by the blending of the genetic elements, 

 effected by the union of two, when both are in an amoeboid 

 condition, from which a still sac results, in which germs or 

 sporules are formed, which eventually escape, and again 

 originate the life-cycle." 



Drs. Dallinger and Drysdale have shown that the in- 

 vestigations of Bastian, Gros, and others, on heterogenesis 

 and the transformation of living forms, are erroneous. They 

 state that as far as their researches on the Monads go, they 

 are bound to say that not the slightest countenance is given 

 to the doctrine of heterogenesis. " On the contrary, the life- 

 cycle of a Monad is as rigidly circumscribed within defined 

 limits as that of a mollusc or a bird. There is no indication 

 of any unusual or more intense methods of specific mutation 

 than those resulting from the secular processes involved in 

 the Darwinian law, which is held to furnish the only legitimate 

 theory of the origin of the species." 



The Infusoria tentacidifera multiply by (i) longitudinal 

 fission, (2) "the development of ciliated embryos in the interior 

 of the body. These embryos result from the separation of a 

 portion of the endoplast, and its conversion into a globular 

 or oval germ, which, in some species, is wholly covered with 

 vibratile cilia, while in others, the cilia are confined to a zone 

 around the middle of the embryo. The germ makes its 

 escape by bursting through the body wall of its parent." 

 This free swimming organism rapidly assumes the adult 

 form. (3) Conjugation takes place in these organisms, which 



* Transactions of lioyal Microscoj^>ical Society, 1875, p. 195. 



t In the case of Tetramitus rodratuH, fission proceeds for from six to 

 eight hours ; and in that of DaUiuf/cria DrysdaJi there are from seven to 

 eiglat acts of fission in an hour, for the first four hours, and about five per 

 hour during the next two hours, after which acts are performed at longer 

 intervals. 



