OVUM. 



ual production ; the second as coming under 

 the category of alternate generation ; and the 

 Vorticella embryo of the Acineta-fonn either 

 repeats its geninial multiplication, or becomes 

 encysted, and gives rise then by its nuclear 

 division to embryonal production. Other 

 new forms of Infusoria are described by Stein 

 under the names Spirochona gemmipara, 

 Dendrocometes paradoxus, and Lngeno- 

 phrys vaginicola, ampulla, and nassa, in which 

 the mode of reproduction is somewhat similar. 



These observations at once show the im- 

 portance of the views entertained by some 

 authors as to the share the nucleus may take 

 in new production, and strongly indicate that 

 much still remains to be known from ob- 

 servation of the processes of reproduction 

 among the Infusoria. 



Should these observations be confirmed, 

 another analogy, in addition to those already 

 observed, will be shown to exist between the 

 organisation and functions of the Protozoa, 

 and those of the lowest plants.* The ten- 

 dency of various other recent researches, to 

 which it has been impossible to refer more 

 particularly in this place, seems to be to show 

 that, in addition to the more common and 

 obvious mode of multiplication by division 

 and gemmation, by which the Infusoria, when 

 vigorous and well nourished, are reproduced, 

 there are other means by which, in dif- 

 ferent circumstances, the more permanent re- 

 production of the species may be secured ; 

 that minute cells are formed within them 

 for that purpose, which may at present be 

 called reproductive cell-germs rather than 

 ova, till a more complete knowledge shall 

 have been obtained of their nature and of 

 the circumstances attending their formation ; 

 and that it is very probable that in the 

 protozoa, as in the simplest plants, the com- 

 bination of the contents of two cells, to all 

 appearance similar, may, as in the process of 

 conjugation, be the necessary preliminary step 

 to the development of the reproductive germs. 



It ought at the same time to be kept in 

 view that the Infusoria may, like many other 

 animals, be subject, some to metamorphosis, 

 and others to alternate generations. Already, 

 since the publication of the great work of 

 Ehrenberg, most important modifications of 

 his system of these animals have been found 

 necessary, and it seems almost certain that it 

 is destined to undergo still farther changes, 

 many of those forms which are now recog- 

 nised as belonging to distinct genera and 

 species being possibly no more than different 

 stages of development of the same animal. 



2nd. Of the possibility of primary, direct, or 

 non-parental production of animals, or of so- 

 called spontaneous and equivocal generation. 



From what has before been stated as to the 

 very general, and almost universal, existence 

 of the sexual mode of generation among ani- 

 mals, and from the reasons that have been 

 given for the belief that in those few and 

 simple animals in which a sexual distinction 



* See the recent work of Alex. Braun, entitled 

 Die Verjungung in der Xatur, Freiburg, ISil). 



has not yet been ascertained, there may still 

 be propagation by means of minute germs, 

 the reader will already have drawn a con- 

 clusion as to the very insufficient nature of 

 the proof that can now be adduced in favour 

 of the view that certain animals may arise 

 independently of pre-existing individuals of 

 the same species. The hypothesis might, per- 

 haps, be at once dismissed with the remark 

 of a recent writer*, " that it is safer to trust 

 to generally prevailing laws, than to confide in 

 such of our observations as are contrary to 

 them." But as in the article GENERATION f , 

 the author was led by a careful examination 

 of the evidence then available on the subject, 

 to admit the probability of the non-parental 

 mode of production as an exceptional occur- 

 rence, at least among the lowest tribes of 

 animals and plants, and as that hypothesis has 

 since gradually lost more and more of its pro- 

 bability, from the accumulated opposing proofs 

 resulting from more recent researches, so as, 

 in Ins opinion, to be now no longer tenable, it 

 may be proper at this place to review briefly 

 the bearing of the present more advanced 

 knowledge "of the generative process upon 

 this long" and keenly debated question. 



Admitting, in the meantime, that the ova, 

 or separate germs of Infusoria, have not yet 

 been discovered with certainty, there are not 

 wanting direct experiments which demonstrate 

 that in an infusion of organic matter which 

 would, when exposed to the air, naturally 

 furnish a rapid succession of these produc- 

 tions, the development of living organisms 

 is entirely suspended, if the arrangements 

 are made such as to render it impossible 

 for any germ or other part of a previously 

 existing inf'usorian animalcule or plant to be 

 communicated to the infusion. The experi- 

 ments of Schultze and of Schwann are 

 valuable, as appearing to have secured, in 

 a <*reat measure, the above-mentioned con- 

 ditions, without otherwise interfering with the 

 validity of the result. The first of these ob- 



Fig.6. 



Apparatus employed by Schultze to prevent the access 

 of germs by the air to an infusion. 



a, ilask for infusion ; b, tube, with caustic pot;isli ; 

 c, tube, with sulphuric acid. 



* Eschricht, in Edinr. New Phil. Journ. vol. xxxi. 

 18-11. p. 355. 

 t P. 429. 



