246 Vniversitij of California Publications in Zoology [Vol. 16 



2. ilORPHOLOGY 



In this stage of tlie amoeba the general form is elongated with the 

 length 1.5 to 2.5, rarely equalling 5 times the diameter. The large 

 diameter varies from 6 to 46/.<, (pi. 19, figs. 19, 27) ; the transverse 

 diameter in the different individuals varies from 4 to about 11.5/x. In 

 a young culture, one in which the amoebas have not divided very many 

 times, the average size is larger (pi. 18), but after numerous divisions 

 have taken place there is a decrease in size of the individual and like- 

 wise of the nucleus. Therefore a young culture is much more satis- 

 factory for the study of mitosis. 



In form the trophozoite is characteristically of the limax type. It 

 has blunt, rounded ends, but its proportions vary in different indi- 

 viduals, in the same individual from time to time, and so on. It may 

 have its outline uneven; or it may be long and .slender, the length 

 being more than twenty times as great as the smallest diameter. Or 

 again it may be shorter and thicker, having the greatest diameter 

 only about three times that of the smallest, which, in a given individual, 

 varies very little. 



"When the animal is at rest, the ectoplasm is not differentiated from 

 the endoplasm (pi. 18, fig. 10) but when it is moving, the pseudo- 

 podium is largely of ectoplasm which is clear and homogeneous, while 

 the endopla.sm is granular (pi. 19, fig. 19). "With neutral red there are 

 rather numerous large granules in the endoplasm which take the stain. 



A contractile vacuole is present. It is formed by the fusion of three 

 or four small vacuoles. Its position is posterior to the nucleus in the 

 moving form. The time from one contraction to the next is usually 

 from forty to sixty seconds. Food vacuoles lie in the endoplasm and 

 are filled with bacteria in the. process of digestion. The bacteria found 

 in the cultures come partly from air contamination, but some come from 

 the soil being inoculated into the culture with the amoeba. These 

 serve as food. Some amoebas may be found very full of food vacuoles, 

 but ordinarily there is only one or few (pi. 18, fig. 13). Quite often 

 none are found. 



The resting nucleus, which is spherical except in very much elon- 

 gated individuals (pi. 19, fig. 19), will average two to four microns in 

 diameter. There is a distinct, not a double-contoured, nuclear mem- 

 brane, but at the periphery of the nucleus is " eosinophile " chromatin 

 (Chatton and Lalung-Bonnaire, 1912). This chromatin may be barely 

 observable (pi. 19, fig. 19) or very evident (pi. 20, fig. 40), being in the 



