AUXETICS 39 



ordinary round type. In a 0'1-per-cent. citric-acid 

 solution, the long forms were killed outright. 

 This type has since occurred on about half a 

 dozen occasions, always accompanied by the 

 minute amcebulae already mentioned, and always 

 under conditions adverse to growth, such as 

 deficient bacterial growth on excessively acid or 

 alkaline media, lack of moisture, etc. They have 

 appeared in cultures made from a single cyst kept 

 under sterile conditions, so they are obviously 

 not adventitious forms which have contaminated 

 the cultures. By the method of isolation described 

 later, on eight occasions one of them has been 

 planted on a jelly medium along with bacteria, 

 but has never multiplied (fig. 16). The inter- 

 pretation we put on their origin is that they 

 are derived from amcebulae, which under the 

 adverse conditions present are unable to develop 

 into the typical adult form. It is interesting to 

 note that in various organisms deficiency of food 

 has been shown to predispose to the formation of 

 the male sex of a species, and it is possible that 

 here we have an example of an attempt at a 

 differentiation of sex. 



Tyrosin (auocetic}. Cysts placed on a jelly 

 film saturated with tyrosin (3-per-cent.) produce 

 free forms in twenty-four hours, which become 

 larger, and contain more multinucleate types 

 (fig. 17) than on an ordinary plain 2-per-cent. 

 agar medium. It is interesting that tyrosin 

 has a somewhat similar effect on certain 

 bacteria, as a pure culture of Bacillus fluorescens 

 Uquefaciens on media containing this auxetic 

 exhibited very large forms of the individual 



