64 GENERAL HISTORY OF 



Section XXIII. — Reproduction of PoJygastrica. 



Monas vivipara is the only species of this class that is 

 viviparous, though some moving granules observed amongst 

 the Bacillaria have been supposed to extend this condi- 

 tion. With this exception, they may be termed oviparous, 

 though besides the formation of eggs, which is a very fer- 

 tile mode of increase, they also propagate, by means of a 

 self- division of the body of the animalcule, into two or 

 more individuals ; also, by the growth of gemmules, or buds, 

 upon the parent. These various modes of propagation 

 account for their almost incomprehensible increase of 

 number in a very short space of time, and which has 

 often astonished observers. 



In the genus Closterium, the curious formation of double 

 gems has been observed by Ehrenberg, and is figured in 

 plate I. fig. 67. That observer remarks, that this accounts 

 for " the astonishing great fertility or capacity of increase of 

 microscopic animals, according to which an imperceptible 

 corpuscle can become, in fovir days, one hundred and seventy 

 billions, or as many single individual animalcules as con- 

 tained in two cubic feet of the stone from the polishing 

 slate of Bilin. This increase takes place by voluntary 

 division, and this is the character which sej^arates animals 

 from plants. It is true that the gemmation in plants, 

 especially, in very simple cells, is at times very similar to 

 the division in animals ; but this relates to the form, not 

 the formation. A vegetable cell, apparently capable of 

 self-division, always became one, or contemporaneously 



