Size in Relation to Body Surface 169 



the medium, except in the first few seconds of treat- 

 ment. Moreover, the effect of soaps and the like upon 

 liquid :air surface is hardly equalled by their effect 

 upon liquid : liquid surfaces. 



Paramecia were reported by Bauer ('24) to swell 

 when sodium taurocholate was added to the culture 

 fluid. No measurements were recorded in support of 

 the conclusion, and rounding-up of the bodies was ob- 

 served, which would make it difficult to compare ac- 

 curately the total body volumes under the two condi- 

 tions. 



Perhaps the only striking evidence for the import- 

 ance of surface forces is the generally observed fact 

 that fission occurs in the equator of least area. An 

 elonguated body is usually fissioned at right angles to 

 the long axis. There are some apparent exceptions to 

 this rule, which rule originated with Hofmeister ('63). 

 The exceptions may be taken to indicate that surface 

 forces are not in every case the most intense ones 

 which are operative. 



In a number of unicellular species it has been ob- 

 served that the growth of individuals is almost wholly 

 in length, and is not equal in all directions. Parame- 

 cium, and certain, perhaps many, bacilli are examples 

 (Jennings, '08b; Schmalhausen '26; Henrici, '28, p. 93). 

 The ratio of surface to mass is thus kept almost con- 

 stant between one cell division and the next, and so 

 the total energy of the surface keeps pace with the 

 mass. Unless, therefore, the tension in the surface 

 varies with absolute size, there is no corollary of simili- 

 tude which would cause a crisis to bring on fission. At 

 present there is nothing to show why growth is dispro- 

 portionate in the various dimensions. Reasons need 

 urgently to be discovered as to why most kinds of uni- 

 cellular organisms are not spherical, and why even 



