WITH ANIMAL PHYSIOLOGY. 113 



sence of nitrogen in the one case and its absence 

 in the other (45). 



80. Perhaps, however, the most curious and 

 interesting analogy between animal and vege- 

 table organization is that which relates to the 

 process of reproduction which in some of the 

 lowest tribes of animals approaches more nearly 

 to identity with that of plants than in any other 

 function. In several of the most minute of 

 the Infusoria, in which nevertheless, small as 

 they are, the patient investigation of Ehren- 

 berg has discovered a series of stomachs, we 

 meet with frequent examples of multiplication 

 by the spontaneous division of the body of the 

 parent into two or more parts. " Many species 

 of Monads for instance, which are naturally of 

 a globular shape, exhibit at a certain period of 

 their development a slight circular groove round 

 the middle of their bodies, which by degrees be- 

 coming deeper, changes their form to that of 

 an hour-glass; and the middle part becoming 

 still more contracted, they present the appear- 

 ance of two balls united by a mere point. The 

 monads in this state are seen swimming irre- 

 gularly in the fluid ; as if animated by two dif- 

 ferent volitions ; and apparently for the purpose 

 of tearing asunder the last connecting fibres, 



