560 GENERAL PHYSIOLOGY 



protoplasmic masses. These simplest of all contractile phenomena 

 offer insuperable difficulties to Engelmann's view. In order to 

 bring the phenomena of amoeboid movement into harmony with his 

 theory, Engelmann (79, 1) is forced to the assumption that in amoe- 

 boid protoplasm also the contractile elements have an elongated 

 form and are capable of swelling so as to become spherical. But 

 this assumption made ad hoc is not only not based upon facts, but 

 is unable really to explain the phenomena. In spite of careful 

 investigation Engelmann has not succeeded in finding in amoeboid 

 protoplasm doubly-refracting elements similar to the fibrous struc- 

 tures of muscular substance. The observation that in Actino- 

 sphcerium the pseudopodia have a doubly-refracting axial strand, is 

 not applicable, because this axial strand has nothing whatever to 

 do with contraction ; it is simply a track upon which the contrac- 

 tile protoplasm can flow, and hence is analogous to the rays of the 

 radiolarian skeleton, which are very wide-spread, especially in the 

 Acanthometridce. But, even if the contractile protoplasm of Rhizo- 

 poda consists of numerous elongated elements that become 

 spherical upon swelling, the extension of the extraordinarily long 

 and slender thread-like pseudopodia that characterise most 

 Foraminifera and Radiolaria and numerous fresh-water Rhizopoda, 

 would be wholly inconceivable upon this assumption. These 

 varieties of pseudopodia are formed simply by the extension of the 

 shorter, blunt or incised processes of an Amoeba or a leucocyte. 

 Even the formation of these latter pseudopodia cannot be explained 

 according to Engelmann's view. How is the occurrence of even a 

 moderate change of form of the Amceba body to be imagined through 

 the simple extension of numerous elements which are of a size far 

 below the limit of perceptibility and, as Engelmann himself 

 assumes, lie irregularly among one another pointing in all direc- 

 tions ? These difficulties are insurmountable. 



We have here arrived at the point where the problem of the 

 movements of contraction can first be taken into consideration with 

 reference to the result. In the amoeboid cell there is the most primi- 

 tive form of contractile substance ; here the relations are undeniably 

 much simpler than in the fibrous forms with their complex 

 differentiations. Moreover, the phenomena exhibited by the living 

 object can be investigated experimentally with incomparably more 

 ease in the free-living and relatively large protoplasmic masses of 

 amoeboid cells, than in the very small constituents of the muscle, 

 which, separated from continuity with their neighbours, invariably 

 perish in a very short time. 



Hence, we will consider, first, the amoeboid movement of naked 



protoplasmic masses. 1 As has already been seen, 2 the element 



common to all phenomena of contraction is the alternation of two 



opposed phases, one of contraction in which the surface is diminished 



1 Cf. Verworn ('92, 1). 2 Cf. pp. 233 and 252. 



