cannibalism in amoeba 679 



4. Description of the Free Sphere. 



One of these recently extruded spheres, observed on July 21, 

 1920, was being rolled over and over by the movements of 

 P a r a m e c i u m b u r s a r i a in the culture, and was seen to be 

 perfectly spherical. The cytoplasm was clear, containing fine 

 dark-looking grains together with some larger refractile 

 granules, the nature of which I have been unable to determine. 

 In stained preparations the cytoplasm showed a well-marked 

 meshwork structure and the fine grains referred to above were 

 distinguishable, being distributed over the strands of the 

 meshwork and especially heaped up around the nucleus 

 (cf. the description of the amoeba, p. 672). 



In the living sphere the nucleus could not usually be distin- 

 guished, but in a few cases I was able to detect it. It is possible 

 that the spheres in which it was visible were dead ones. 



As has been noted above, in stained preparations the nucleus 

 of the sphere shows the same structure as that of the amoeba 

 itself. While some of the spheres contained no other structure, 

 others, on the contrary, were full of diatoms and other bodies 

 in food vacuoles (PL 28, figs. 2, 3, and 5). Sometimes, when 

 the amoebae contained zoochlorellae the spheres also contained 

 them. 



The outline of the spheres was very definite, appearing as 

 a dark line, giving the impression that a definite limiting 

 membrane was present. Examination of stained preparations 

 showed, however, that no such limiting membrane is really 

 there, the effect of a membrane being produced by the arrange- 

 ment of the meshwork structure of the cytoplasm at the surface, 

 an effect which is commonly seen also in rounded-off amoebae. 



In spheres observed under the oil-immersion lens, it was 

 noted that, while immediately after extrusion no contractile 

 vacuoles were present, these appeared a short time after extru- 

 sion. In no case have contractile vacuoles been seen in the 

 spheres while they are still in the amoebae. They were never 

 present when the spheres were extruded, but they often 

 appeared soon after extrusion. Since their appearance is at 



