AMEliOllt MOTION OF LEUCOCYTES 275 



changes in surface tension, yet we iind in some protozoa that pseudo- 

 podia may take on the persistence and action of cilia, and that cilia 

 may seem to chanfje into pseudopodia. Jennings has made a most 

 extended stndy of the relations of tlie "liehavior of Lower Organ- 

 isms""- to the physical theories of ameboid motion, and is unable 

 to corroborate the claim that the processes that go on in "artificial 

 amebffi" exactly reproduce those of living amebaj, or to accept the 

 statement that living ])rotoplasm behaves exactly as any similar drop 

 of fluid would under the same conditions. He states tliat the currents 

 set up in artificial amebfe by changes in surface tension are not the 

 same as those in living amebae, contrary to Rhumblei* and to Biitschli. 

 The movement of ameba, he maintains, is not due to the flowing of 

 the contents of tlie cell in a central, axial current out into the pseudo- 

 podium and back on the sides, as occurs in the artificial ameba; but 

 rather to a rolling forward of the upper surface over the anterior 

 edge to the lower surface, where it becomes fixed to the surface on 

 which the ameba is crawling. Tlie part played by surface tension, 

 he claims, is in the case of ameba? a very subordinate one, and it is 

 not sufficient to explain the movements of the living cell. 



However the discussion concerning the amebse may turn, it must 

 be appreciated that there are some important diff'erences between even 

 the ameba and the leucocyte. The latter has by far the simpler 

 organization, and approaches in structure, and presumably, therefore, 

 also in response to stimuli, more closely to the simple drop of colloid 

 matter. It has no pulsating vacuoles, no specialized pseudopodia, 

 never forms shells or coverings, and does not conjugate as do the 

 amebffi. The extenial surface of the leucocyte is much simpler, an 

 important fact in connection wdth surface tension etfects, for in the 

 leucocyte the surface seems to be practically undifferentiated, naked 

 protoplasm; whereas in amebae it is formed of a well-differentiated 

 "ectosarc, " which has marked motile powers, being able to contract 

 sufficiently to cut an injured ameba completely in two. At the very 

 least the surface tension explanation of leucocytic action agrees per- 

 fectlji with most of the ohserred actions of leucocjjtes, and it is the 

 only reasonable theory offered. There seems to be no middle ground 

 between such a physical theory and a metaphysical theory which 

 would endow a single cell, without organs or nervous system, with the 

 reasoning powers of highly developed animals, a position incompati- 

 ble with the entire evidence of experience. 



62 Publication No. 16, Carnegie Institute, Washington, 1904; also see American 

 Naturalist, 1904 (38), 625. 



