84 LIBBIE H. HYMAN 



cosa, and possibly some other species. As Dellinger ('06) first 

 showed, Amoeba proteus and other rhizopods are attached only 

 at limited points, the tips of the pseudopodia, and generally the 

 posterior end also. Pseudopodia are first extended free into the 

 water, then attach at their tips with more or less curvature, then 

 contract. The amoeba 'walks' on the tips of its pseudopodia. 

 This behavior seems to me completely incompatible with the 

 idea that the amoeba is fluid and compels the conclusion that the 

 ectoplasm is more or less rigid. I may say that there is no doubt 

 that both of the varieties of Amoeba proteus which I have used 

 move exactly in the way described by Dellinger. This is 

 readily determined by careful focusing, by passing a needle point 

 under the animal,, etc. without the use of the horizontal method of 

 observation. I have frequently observed the animals 'walk- 

 ing' on the under side of a cover glass with the tips of their 

 pseudopodia flattened against the glass, while the central mass 

 hung free in the water. The tips of the pseudopodia cling to the 

 substratum with such force that strong squirts from a pipette 

 are required to detach them, and the amoebae may be pulled 

 in two, as described above, before they will loosen their hold. 

 If a moving amoeba is quickly detached, and rolled about with 

 a needle, the form, shape and curvature of the pseudopodia can 

 easily be determined, for these are retained for some time, and 

 one can often even see the flattened plane surface of the pre- 

 viously attached pseudopodial tip, which persists for a little while 

 in this condition in defiance of all the laws of minimum surface! 

 Anyone who makes these observations can have no particle of 

 doubt regarding the existence of solid properties in the ectoplasm. 

 I believe, however, that Dellinger's discovery while practically 

 proving the contractile nature of the ectoplasm, though no more 

 so than do Jennings' numerous and careful observations, does not 

 justify his assumption that a 'contractile substance' is present in 

 the ectoplasm, nor his comparison of amoeboid motion with that 

 of an animal like the leech which is provided with contractile 

 fibers. As Rhumbler in his reference to Dellinger ('07) has justly 

 remarked, nothing is gained by referring amoeboid motion to 

 contractile elements, since what we are trying to do is to explain 



