THE NATURE OF REPRODUCTION. 579 



detected in its contents. But they showed themselves in abundance 

 after the flask had been opened, and the infusion exposed for a few days 

 to the direct access of air. 



Pasteur* found that if a flask containing an organic infusion were 

 boiled upon a high mountain, where the air is of unusual purity, allowed 

 to fill itself with this air while cooling, and then hermetically sealed, it 

 would often remain free from infusorial growth. Several such flasks, 

 boiled and filled with air on the Montanvert in Switzerland, were kept 

 for four rears, without their contents undergoing any perceptible 

 change. But on making, at the end of that time, a minute opening in 

 the neck of one flask, the infusion exhibited after three days a percepti- 

 ble growth of cryptogamic vegetation. 



These results did not absolutely exclude the possibility of spontane- 

 ous generation, which was still maintained by Pouchet and other ob- 

 servers ; but they indicated the existence of atmospheric germs, capable 

 of development in a suitable organic infusion. 



But in the mean time the study of the infusoria had been going on 

 independently of the question of spontaneous generation, and had been 

 sufficient to demonstrate their reproduction in the usual way, by fer- 

 tilized eggs and embryonic development. 



The apparent confusion and variability in form of the infusoria, at 

 the time of their fir.st discovery, depended largely on their great num- 

 bers and imperfect knowledge of their natural history. Subsequent 

 observation has shown their organization to be as definite as that of 

 other classes of the animal kingdom ; and they have now been arranged, 

 by the labors of Claparede and Lachmann,f Stein, J and Balbiani, in 

 orders, families, genera, and species, which may be recognized with 

 certainty by their distinctive marks. They are not confined to infusions 

 of decaying material ; but many have their natural habitation in lakes, 

 pools, marshes, running brooks, or the open sea. Certain forms, origi- 

 nally included in this class, such as Rotifer, Stephanoceros, and Flos- 

 cularia, have been found to possess a more complicated structure than 

 the rest, and to belong properly to the class of worms ; their mode of 

 reproduction being sufficiently manifest from the fact that they often 

 contain living embryos, in process of development. 



Finally, the true ciliated infusoria have also been shown to reproduce 

 their species by means of eggs, formed in special generative organs and 

 fecundated by union of the sexes (Fig. 156). This fact, first demon- 

 strated by Balbiani, has been since confirmed, in numerous instances, 

 by Stein, Engelmann,|| and Cohn ;^[ Balbiani and Stein together having 



* Comptes Eendus de 1' Academic des Sciences. Paris, Fevrier 20, 1865. 



f Etudes sur les Infusoires et les Rhizopodes. Geneve, 1856-1861. 



+ Organ ismus der Infusionsthiere. Leipzig, 1859. 



\ Journal de la Physiologic de 1'Homme et des Animaux. Paris, 1861. 



|| Zeitschrift fur wissenschaftliche Zoologie. Leipzig, 1862, Band xi., p. 347. 



V Ibid. Band xii., p. 197. 



