934 SPECIAL PHYSIOLOGY. r , 



placed so as to intercept the germs in the air ; this gun-cotton being 

 afterwards dissolved in ether, ova and germ-forms were collected in 

 the residue, and recognized under the microscope. Moreover, portions 

 of the cotton-wool or gun-cotton charged with the germs, produced, in 

 water or in vegetable infusions, the same kinds of Infusoria as ap- 

 peared in the liquids unprotected by the gun-cotton. 



It would seem, however, that in boiled vegetable infusions, hermeti- 

 cally sealed with a certain quantity of air, also previously heated to a 

 red heat, those singular organisms, named bacteria, which are proba- 

 bly vegetable, may, after several months, appear. (Child.) 



The position of the advocates of the doctrine of spontaneous gener- 

 ation is a difficult one. An apparently positive result, in an experi- 

 ment with hermetically-closed vessels, is attributed by their opponents 

 to want of care in the preparation of the water or the infusion, or to 

 the accidental intrusion of germs. The position of the opponents of 

 the doctrine is also difficult, because they seek only to establish a 

 negative. A sufficient number of negative results, obtained by con- 

 scientious observers, is all the evidence that can possibly be advanced 

 in such a case. The onus probandi is thrown upon the supporters of 

 the doctrine. 



The Various Modes of Reproduction. 



In the Vegetable Kingdom, two modes of reproduction are observed, viz. , 

 the non-sexual and the sexual. The former presents several varieties, vi^, 

 gemmation, or the formation of buds, as in ordinary plants, or in special cases, 

 of bulbs which are detached buds, or subdivision or fission, as in the micro- 

 scopic algae, and perhaps in the lower fungi. The sexual mode is by true 

 spores or seeds, which require fertilization. The two modes commonly occur 

 in all plants. 



The mode of propagation by buds, or by cuttings, serves to prolong a variety, 

 or a species, merely by multiplying the individual ; but the sexual mode alone 

 will render permanent an accidental variety, or will perpetuate, for a length 

 of time, a specific form. In the fungi, and in the lower algaceous forms, it 

 seems probable that an alternate form of generation may occur, such as will 

 presently be described in the case of certain animals : that is to say, the two 

 forms of reproduction, sexual and non-sexual, may alternate in different gener- 

 ations ; in other words, spores may produce intermediate forms, which, in 

 their turn, may directly reproduce the parent form. 



The reproduction of animals from parents, also presents the non-sex- 

 ual and the sexual modes. 



The non-sexual mode of reproduction may either consist in a simple 

 cleavage or division called fission, or in the formation of buds, known 

 as gemmation or budding. Both these forms occur only in the lower 

 'Classes of animals. Fission, or fissiparous reproduction, consists of a 

 ^constriction, once or several times repeated, in the soft body of an 

 animal, followed by its complete division into two or more parts, each 

 -of which is then developed into an individual as complete, in every 

 respect, as the parent animal. This form of reproduction is noticed 

 as one mode of development in the Infusorial animalcules, the process 

 being sometimes, as in Paramecium, extraordinarily rapid. It also 

 occurs in the formation of the segments in some vermiform intestinal 



