250 The Philippine Journal of Science i9is 



success on the part of these organisms in avoiding being carried 

 away. This apparently constitutes an application of the prin- 

 ciple of the "avoiding reaction" as described by Jennings (24) 

 in his studies on the tropisms in the Protozoa. In the smaller 

 vessels of the intestine, aside from the obstruction afforded by 

 the walls of vessels having a caliber smaller than the diameter 

 of the parasite, the flow is probably too slow to bring about the 

 degree of stimulation necessary to excite this reaction. The 

 parasites would probably be prevented by the excessive blood 

 pressure from even entering the arteries, but granting that they 

 might do this, the stimulation would probably pass above the 

 optimum, and the organisms would be passively swept back. 



But in the larger radicles, where the current is away from 

 the intestines, it is conceivable that the flow would be of about 

 the right intensity to cause the parasites to about face and turn 

 "upstream." At the same time it would be strong enough to 

 sweep the entamoebas 'toward the liver. If this hypothesis will 

 hold, it would be added evidence to the effect that in hepatic 

 involvement the main path would be through the portal circula- 

 tion. Lastly, however, it must not be forgotten that there still 

 remains the possibility that the balantidia may reach the liver, 

 but find the conditions for continuation of life unsuitable at that 

 site. If Stockvis's observation is correct, that contingency 

 seems to be ruled out. The possibility of the conveyance of the 

 cysts of Balantidium to the liver through the blood stream ap- 

 pears to be too remote for consideration at this time. There 

 is no definite proof that Balantidium forms cysts in the tissue, 

 and it would require to be shown that the cysts would open 

 under the influence of any agent that is likely to be present in 

 the liver. Normally these cysts may be expected to open under 

 the influence of some digestive ferment only, though it is well 

 known to protozoologists that certain protozoan cysts will open 

 under the influence of ferments secreted by bacteria or through 

 external or internal influences of an uncertain nature. The 

 cysts of the intestinal parasites, however, appear to be adapted 

 to the action of the intestinal juices. 



Turning back to the flagellates, let us review what evidence 

 we actually have of their tissue-invading powers. 



Minchin and Thomson, in their great memoir on Trypanosoma 

 lewisi and its relations to the rat flea Ceratophyllus fasciatus,(36) 

 describe in considerable detail an intracellular stage in the 

 stomach of Ceratophyllus following the penetration of the epithe- 

 lium. They were not so fortunate as to observe the actual 



