304 PROTOPLASM 



mixed with the water. If the layer of water is fairly 

 shallow, but not so much so as to be broken through by the 

 depression, and if the black has already collected to some 

 extent upon the bottom, it may be observed, on approximat- 

 ing a rod dipped in ether, that the colour is completely 

 removed from a ring -like zone corresponding to the 

 periphery of the rod, and at its inner edge is densely massed 

 in a black ring immediately under the rod. That is to say, 

 by the vortex current at the bottom of the vessel returning 

 towards the rod the colouring matter is all carried inwards, 

 and remains lying at the spot where this vortex current 

 ascends upwards again. 



As to .the explanation of the retreating movement, however, 

 it results from these observations that the retreat must depend 

 upon the back currents in the drop being in excess of the 

 forward currents ; but this can only be due to the adhesion of 

 the drop, which offers a hindrance to the forward current which 

 goes on in the deeper layer. As a proof of this fact there 

 is also the observation, which has already been mentioned 

 above, that the retreating movement is especially pronounced 

 when the drops are very shallow. In any case, however, 

 all these observations confirm the view that the movements 

 of Amoebae cannot take place in this way. 



Berthold, however, is willing to admit, as has been 

 already remarked above, that pseudopodia are occasionally 

 sent out by forces acting from within, somewhat in the 

 same manner as was supposed by the older observers, 

 namely, by contraction of the hinder part of the body of the 

 Amoebae (p. 102). Contractions of this kind may be pro- 

 duced by changes in the condition of imbibition of the 

 protoplasm (pp. 102 and 105). Finally, Berthold adds to the 

 three hypotheses already mentioned, concerning the causes 

 of protoplasmic movements, yet a fourth, in order to explain 

 the origin of fine filamentous pseudopodia, which rise up 

 free into their surroundings ; for which no one of the three 

 detailed is sufficient. Since Berthold's ideas of these 

 processes can scarcely be described briefly, on account of 

 the uncertainty of the forces brought into the problem, 

 I refer the reader to his memoir, and will confine my 



