1922] McDonald: On Balantidium coli and Balantidium suis 279 



the ectoplasm of this cone into the endoplasm, each bears an enlarge- 

 ment which stains very deeply. This enlargement is undoubtedly 

 homologous with the inner enlargements of the basal apparatus of 

 the body cilia for it bears the same relation to the basal granule and 

 the cilium. It is farther removed from the basal granule but it main- 

 tains exactly the same relation to the ectoplasm and endoplasm, that 

 is to say, in either case this enlargement lies in the plane between the 

 two. From each enlargement a radial fiber extends laterally along 

 the base of the apical cone to the periphery of the cell. The portion 

 of the ciliary rootlet which continues inward from this enlargement is 

 very distinct and may often be traced almost to the posterior end of 

 the animal (fig. I). In no instance has it been possible to demonstrate 

 any attachment or connection of the posterior ends of these rootlets. 

 They often cross near the center of the organism and then fade out, 

 becoming indistinguishable in the endoplasm. For a short distance 

 inward from the oesophagus they are so closely placed dorsally and 

 laterally that they form a kind of wall, but ventrally they are less 

 numerous and likely to be much shorter, so that the pseudo-wall is 

 not complete. It is sufficient, however, to direct the food for some 

 distance into the endoplasm. There is no such high degree of differ- 

 entiation here as Sharp (1914) found in the oesophagus of Diplo- 

 dinium ecaudatum. 



The homology of the adoral and body cilia is evidenced by the com- 

 plete series of gradations from the one to the other which exist in the 

 cilia of the apical cone (fig. I). The cilia which are proximal to the 

 adoral cilia are almost identical with them. They are slightly smaller, 

 but each has the basal granule beneath the pellicle, a fiber connecting 

 this with an enlargement at the plane of differentiation of ectoplasm 

 from endoplasm, and the rootlet extending posteriorly.- The inner en- 

 largement is much smaller and the ciliary rootlet shorter than those of 

 the adoral cilia. These enlargements are connected with the enlarge- 

 ments of the adoral cilia by the radial fibers mentioned above. Passing 

 posteriorly the cilia become progressively smaller, the basal granule 

 and the enlargement closer together (due to the approach of the trans- 

 verse plane of demarcation between ectoplasm and endoplasm to the 

 pellicle), and the ciliary rootlets become progressively shorter and less 

 distinct, until the cilia of the marginal area of the apical cone become 

 identical with the body cilia. This apical cone then shows us a com- 

 plete gradation of differentiation between the body cilia and the adoral 

 cilia, and proves their homology. 



