498 GENERAL PHYSIOLOGY 



tions from this direction, which are produced by spontaneous im- 

 pulses. The essential factor in all directive reactions is, therefore, 

 the assumption of an axial position by the cell-body, and the explan- 

 ation of this is the key to an understanding of the mechanics of 

 these phenomena. We will now study the mechanism by which 

 the axial position is assumed in various types of free-living cells. 



The simplest and clearest relations are, as always, in naked 

 protoplasmic masses, such as Ammba and leucocytes. Let us 

 imagine an Ammba in a spherical form about to move, and an ex- 

 citation of contraction to appear at one spot as a result of a stimulus 

 acting unilaterally (Fig. 246, a). The excitation would be least at 

 the portion of the surface of the sphere opposite the place of 

 stimulation. The protoplasm there would flow out unhindered, 

 while upon the stimulated side the strong contraction would allow 

 no bulging. The protoplasm would, therefore, form a pseudopodium 

 toward the unstimulated side (Fig. 246, b). Thus an Aniceba, which 

 under conditions that are equal upon all sides extends its 

 pseudopodia in all directions and creeps sometimes here and some- 

 times there, would now assume an axially differentiated form (Fig, 



OO 



Fig. 246. — Scheme of axial orientation in an Amaha resulting from an excitation of contraction 

 upon the right side. The thickness of the contour indicates the excitation. The arrows 

 indicate the direction of the creeping. 



246, c), as is the case in that variety called Amceha Umax. Under 

 these circumstances with continual stimulation upon the same 

 side the Amoeba would necessarily creep gradually away from the 

 source of the stimulus, as is the case in the negative chemo- 

 taxis and thermotaxis of Amoeba, Myxomycetes, leucocytes, etc. 

 Vice versa, if the spherical Amceba were to be acted upon on one 

 side by a stimulus that produces a local excitation of expansion, the 

 protoplasm would flow out most strongly toward that side, so that 

 with continual stimulation the Ammba must necessarily approach 

 the source of the stimulus. The positive chemotaxis of leucocytes, 

 Ammba, Myxomycetes, and other naked protoplasmic masses is thus 

 explained. If a stimulation of expansion acts upon one side of an 

 Ammba, and a stimulation of contraction upon the other side, the 

 results of the two must naturally be expressed in like senses, i.e., 

 the Ammba must creep away from the latter side and toward the 

 former. The galvanotaxis of Ammba affords an unusually clear 

 proof of this {Of. Fig. 232, p. 458). 



The mechanism of axial orientation is less complex in those 

 microscopic organisms that do not consist of protoplasmic masses 



