Studies on the Life History of Protozoa. 439 



and 21). The difficulty appears to be that the cortical plasm is 

 incapable of holding the trichocyst threads after expulsion, 

 for the threads may be easily seen as a cloud around the 

 animal immediately after fixation, while the after-treatment always 

 dislodges them in the cultivated forms, but not in the wild forms. 



c. The Macronucleus. The structure of the normal macro- 

 nucleus of ParamcBctum aurelia was described by Hertwig in 1889 

 and the nucleus of P. caudatum agrees so closely with it, that 

 further details are hardly necessary. It is an elongate, ellip- 

 soidal body, usually with a smooth contour and without breaks 

 of any kind save the minute impression made by the micro- 

 nucleus. It frequently lies in a vacuole which is caused by 

 the action of the fixing fluids, for in life the macronucleus is in 

 immediate contact with the endoplasm. The contraction is 

 probably in the endoplasm away from the nucleus rather than a 

 contraction of the latter. Often there is a depression in the 

 macronucleus due to the pressure of the contractile vacuole, and 

 food vacuoles may also press against it, as Wallengren suggests, 

 and distort it out of the normal proportions. 



In its finer structure the macronucleus is granular with the 

 irregular granules densely packed together, giving the appearance 

 of a homogeneous mass. ■ 



d. The Micronucleus. The micronucleus is usually embedded 

 in the material of the larger nucleus, but may be separated from 

 the latter, even in the resting stages, by a considerable distance, 

 while in the dividing stages it is usually separated. Its finer 

 structure consists of a more or less homogeneous mass of chro- 

 matin frequently arranged in lines, while at one end is an accu- 

 mulation of "achromatic" material in regard to which there is 

 some diff"erence of opinion. In size the micronucleus is about 

 1 1 microns, but in the difi^erent phases the size diff"ers so that this 

 characteristic has but little weight. 



e. The Contractile Vacuoles. In the normal individual these 

 are situated in the anterior and the posterior parts of the body, and 

 about one-third of the length of the body from the ends. They 

 are fed by radiating canals which are conspicuous in the living 

 animals. The pulsation is regular as a rule, but this becomes 



