EFFECT OF MEDIA ON SHAPE OF AMOEBAE 567 



The following work is based on the study of 83 different cultures. 

 All the drawings were made with a camera lucida. A Zeiss No. 

 12 ocular and a 16 mm. Spencer lens objective were used. 



EXPERIMENTS 



Agar-agar of 2.5 per cent was very firm and dense. On this 

 medium the amoebae multiplied and spread out over the Petri 

 dishes in an ever-widening circular area of irregular outline. 

 This area was limited by the bacteria on which the amoebae 

 Uve. As these bacteria multiply and spread over the agar, 

 the amoebae follow them. A few amoebae go out beyond 

 them but the place where the bacteria stop is mostly marked 

 by a dense ring of amoebae eight or ten deep. The amoebae 

 on this medium are large, flat, and have few pseudopodia as 

 they are practically surrounded by their food. Locomotion 

 was very slow. Figure 1 shows such amoebae taken from a two 

 day old 2.5 per cent agar medium. 



In striking contrast to these are the amoebae which are grown 

 on the 0.5 per cent agar. Figure 2 shows such amoebae which 

 have been two days on this medium. They are surrounded by 

 bacteria, and are considerably smaller, more elongated, and 

 have small irregular pseudopodia. Figure 3 is of amoebae which 

 have been seven days on 0.5 per cent medium. They show the 

 same characteristics. 



The amoebae grown on the 0.5 per cent medium are quite 

 evenly distributed over the Petri dish. They are never massed 

 together as in the cultures of denser media, but wander freely. 

 Occasionally one finds them forming a ring at the edge where 

 the bacterial growth stops. This is never a dense ring. It is 

 seldom more than three or four deep with amoebae. The 

 difference in the shape and size of these amoebae and those 

 grown on the 2.5 per cent agar medium is due partly to the fact 

 that the bacteria are not so plentiful here and the amoebae must 

 wander farther for their food supply. Why the bacteria do not 

 multiply so readily on this medium is a question. There is the 

 same amount of peptone as in the denser media. It must be 

 that the agar itself furnishes some food which in the 0.5 per 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 22, NO. 3 



