1922] McDonald: On Balantidium coli and Balantidium suis 267 



microns in thickness. The anterior end of the animal, that is, all 

 anterior to a transverse plane which would transect the body at a 

 point % to y 5 of the way to the posterior end, is composed entirely 

 of ectoplasm. In this cone-shaped area is located the cytostome and 

 a large part of the neuromotor apparatus. The protoplasniTof this 

 region seems to be homogeneously granular in fundamental structure.. 

 It stains very deeply with haematoxylin ; so deeply in fact, that in 

 differentiation it is necessary to destain other parts of the body almost 

 completely before this part reaches a degree of transparency suitable 

 for study. With Mallory's connective tissue stain this region also 

 stains very densely, taking on both the brilliant red and the deep blue 

 elements of the stain, in different structures, as will be explained below. 

 This extensive thickening of the ectoplasm at the anterior end is clearly 

 shown in the figures by Leuckart (1861) and has been noted by nearly 

 all who have studied the animal more recently, but I have failed to 

 find any discussion of its significance. This same phenomenon occurs 

 in the Ophyroscolecidae, as pointed out by Sharp (1914) in his work 

 on Diplodinium and by Braune (1913) in Ophyroscolex. In these 

 cases the change seems to be correlated with the high degree of activity 

 and specialization of the anterior end of the animal ; in Diplodinium 

 in connection with its selective feeding, and in Balantidium in con- 

 nection with both feeding and activity of this entire region in pene- 

 trating the mucosa of the intestine. The centering of the neuromotor 

 apparatus in this region gives additional evidence in regard to this 

 question which will .be discussed further in connection with the de- 

 scription of that apparatus. 



Throughout the entire investigation of the minute structure of this 

 animal, I have been unable to demonstrate the presence of any definite 

 plane of demarcation between endoplasm and ectoplasm, such as the 

 " ectoplasmic boundary layer" described by Sharp (1914) in Diplo- 

 dinium, and shown in plate IV, figure 3 of his paper. Many of the 

 fixed preparations used in the search for such a layer were sections 

 of the animal, treated as nearly as possible according to the technique 

 used by him and stained, as were his preparations, with Mallory 's con- 

 nective tissue stain. So far as it was possible for me to determine, 

 any sharp boundary line between ecto- and endoplasm is lacking. On 

 the contrary, they merge into one another and only in a general way 

 can it be said where one terminates and the other begins. 



Prowazek (1913, fig. 2) describes, in Balantidium coll, "ein Art 

 von Zwischenmembran " which appears as wavy lines in optical sec- 



