1922] McDonald: On Balantidium eoli and Balantidium suis 283 



different with respect to size and proportions as noted above (page 

 259), but structurally they are so closely alike that one description 

 will suffice for both. The macronucleus (mac., fig. I, pi. 28, figs. 13 

 and 14) always lies in the endoplasm but otherwise is not^constant 

 in location. Immediately surrounding it is an area in which the endo- 

 plasm is less granular and less dense in appearance, due perhaps as 

 Yocom (1918) has suggested of Euplotes patella to more rapid oxida- 

 tion in this region. It is elongate and may be straight and rodlike 

 or it may be sharply bent into a horseshoe shape. In any case its 

 diameter increases toward either end, giving it something of a dumb- 

 bell shape. This constriction in the central region and enlargement 

 at each end is more marked in Balantidium coli than in Balantidium 

 suis. The nucleus is delimited by a definite nuclear membrane 

 (nuc. m., fig. I) which is especially apparent in material in which 

 the macronucleus has shrunk due to faulty technique. Within this 

 membrane are packed rather densely the masses of chromatin. The 

 chromatin never occurs in equal sized regular granules but rather in 

 unequal very irregular masses, sometimes of considerable size (1 to 2/x, 

 in greatest dimension). Often there is a sort of vacuolated area, 

 usually near one end of the macronucleus, which is free from chroma- 

 tin. The significance of this vacuolated area I could not determine. 

 It does not seem to be due to degeneration and is not related to any 

 phase of reproduction. It is in no way comparable to the ' ' reconstruc- 

 tion band" described by Griffin (1910) and by Yocom (1918) in 

 Euplotes worcesteri and Euplotes patella, respectively. The chromatin 

 stains black with haematoxylin so that the macronucleus is the most 

 conspicuous structure in a stained individual. When Mallory's con- 

 nective tissue stain is used, the macronucleus takes on an orange hue. 

 Micronucleus. The micronuclei (mic., fig. I) of both species of 

 Balantidium parasitic in pigs are exceedingly small, not exceeding 

 5/x. in diameter. In the resting or vegetative phase the micronucleus 

 is subspherical and its flattened side lies close against the nuclear 

 membrane of the macronucleus. It may even lie in a depression in 

 the macronucleus in which position it is scarcely distinguishable from 

 the granules of the latter. It is surrounded by a nuclear membrane 

 readily recognized when the micronucleus is undergoing mitosis. In 

 a few instances the chromatin has appeared indistinctly granular but 

 it is customarily so closely packed that it looks like a single solid mass. 



