EFFECT OF MEDIA ON SHAPE OF AMOEBAE 571 



Here we have the greater freedom of movement and the result- 

 ing lengthening out of the amoeba and the formation of pseudo- 

 podia. This is even more strikingly shown in the case of the 

 amoebae transferred to water (fig. 7). 



On the thicker media the waste products do not diffuse so rap- 

 idly and consequently this relation between the cell plasm and 

 medium is not so easily obtained. It is therefore to be ex- 

 pected that we should not find the amoebae here wandering so 

 freely nor having so many pseadopodia. This is the case. We 

 find on the denser media the cells are massed together in great 

 groups. They are surrounded by bacteria, so have plenty of 

 food for growth. They divide and multiply rapidly, with only 

 those at the edge wandeiing out into the medium. The great 

 accumulation of waste material must liquefy the medium so that 

 those farthest from the mass of amoebae can move freely out- 

 ward, having here made for themselves the same relation between 

 the cells plasm and the medium as the amoebae on the 0.5 per 

 cent agar medium normally have. When such a culture, which 

 has divided rapidly, is two or three weeks old, depending on the 

 rate of division and the rate of metabolism, we find the amoebae 

 all over it are spindle shaped and elongated (fig. 8) showing 

 that the medium has been sufficiently liquefied by the waste 

 products to allow this freedom of motion. We also note that 

 many of the cells are smaller (fig. 9), which is no doubt due to 

 the lack of oxygen, as suggested by Burrows ('13). 



Later, however, when the accumulation of waste products is 

 very great, and the amoebae have multiplied so fast that there 

 is no further place for them to go, they encyst. I have frequently 

 observed a culture dish containing thousands of amoebae, with 

 plenty of bacteria as food, soon become covered with cysts, the 

 amoebae having often encysted over night. When these amoe- 

 bae are transplanted to a new culture medium they come out 

 of their cysts (Hogue '14) and are soon multiplying and spread- 

 ing over the new medium, where the oxygen supply is plentiful 

 and where the waste products have not yet accumulated. 



