226 Dr. Wallich on the Amcebse. 



o/'Amceba. — In one place, as will have been seen, Dr, Gruber 

 expresses himself somewhat doubtfully on this subject. He 

 says : " Upon what law this power depends cannot be stated 

 definitely ; very probably, however, different conditions of 

 pressure come into play in the matter." In a later part of his 

 remarks his doubts would seem to have already been resolved ; 

 for he there states unconditionally that " the pushing forward 

 of the more fluid constituents is effected by the action of a pres- 

 sure on the opposite side," &c. I confess my inability to under- 

 stand in what way " different conditions of pressure " can be 

 developed, if such pressure be not an inherent vital attribute 

 of the sarcode-body itself, manifesting itself by the produc- 

 tion within its own substance of contractility and extensibility. 

 These effects are manifest in the Amoebce in a very high 

 degree, and we know well enough that no pressure of any 

 ordinary kind could actually compress a fluid or semifluid 

 substance like sarcode, even in the slightest degree. 



But, without any argument derivable from hydraulics or 

 hydrostatics, the vital, and, in this sense exceptional, cha- 

 racter of the force that causes a pseudopodium to be projected, 

 and, still more notably, a pseudopodium to be retracted, ap- 

 pears to me to be conclusively demonstrated by what happens 

 during the change from the extension of a pseudopodium in 

 one direction, to its retraction in the opposite direction. As- 

 suming that due precautions are observed to prevent illusory 

 effects, it will be seen that whereas the commencement of the 

 motion of the granules (which is the initiatory step in the 

 projection of a pseudopodium) is distinctly observable at the 

 most advanced portion of the mass which is going to constitute 

 such pseudopodium, the commencement of the retrograde 

 movement of the particles is to be seen taking place at that 

 portion of the pseudopodium which constitutes, not its apex, 

 but its base, each consecutive tier of granules pursuing the 

 same order until all are again reintroduced into the general 

 mass. In short, the order pursued by the granular units in 

 the projection and retraction of a pseudopodium is identical 

 with that pursued by a mass of human units when streaming 

 into an enclosed space through a single door, the human units 

 in front of the mass being naturally the first to flow in, even 

 when those in the rear have the civility not to exert pressure 

 on them ; whereas in flowing out, the units who constituted 

 the rear would be the first to commence the retrograde move- 

 ment. 



But to carry the case a point still further. Assuming, for 

 argument's sake, that " pressure ■" acts, as urged by Dr. 

 Gruber, by " extracting water " from a given portion of the 

 sarcode-body of Amoeba, the explanation would in no wise 

 account for the collapse as well as the inflation with fluid of 



