On the Growth of certain Fresh-water and Soil Protista. 157 



In cultures containing 1-2 c.c. of the tyrosin solution, it was found that 

 after a period of about six to eight weeks the Euglena ceased its active 

 growth and became encysted upon the walls of the tube and especially round 

 the surface. For example, a tube containing 4 c.c. Miquel solution, 6 c.c. 

 tap water, and 1 c.c. tyrosin solution, was inoculated with Euglena on 

 November 25. By December 3 there was a noticeable growth, which 

 became very thick by December 30. By January 30 all free-swimming 

 forms had disappeared, and nothing remained but a ring of encysted 



Fig. 1. — Photograph to show the Growth of Euglena in a Tube (B), containing the 

 optimal Miquel mixture, to which tyrosin solution was added, as described. 



Tube A shows the slight growth in a control culture containing the optimal Miquel 

 solution, but with no organic solution. 

 Photograph shows a 10-days growth. 



Euglena. It was found that by replenishing the culture medium in this 

 tube, the growth of the Euglena could at once be revived : within two days 

 after replenishment a thick growth of free-swimming forms was obtained. 



This suggested that the tyrosin was used up after a certain period. To 

 test this hypothesis, cultures were made in the optimal Miquel mixture, to 

 which tyrosin was added in the solid form, so that as soon as the dissolved 

 tyrosin was used up, fresh tyrosin might go into solution. Cultures grown 

 in this medium showed a very rapid growth of Euglena during the first 

 fortnight or three weeks, but after that the increased development of 



N 2 



