THE SARCODINA 83 



tonema, where there is an elbow or joint which can be bent at right 

 angles. 



No entirely satisfactory explanation of pseudopodia formation and 

 movement has yet appeared, although the subject has been attacked 

 on many sides, and by almost all students of the Rhizopoda since the 

 time of Dujardin. Like the early attempts to explain other phe- 

 nomena in the Protozoa, the first explanations of pseudopodia-motion 

 were based upon the analogy to higher forms. Protoplasmic 

 contractility, the basis of locomotion in all higher animals, and 

 probably in many Protozoa (Mastigophora and Infusoria), was 

 early suggested as the cause of the protrusion of pseudopodia. The 

 majority of casual observers were content with this general explana- 

 tion ; others, more definite, conceived the seat of contraction to be in 

 the cortical plasm or ectoplasm (Ecker, '49; Dujardin, '41), which they 

 compared with the dermal musculature of worms, and which the)' 

 supposed forces out the pseudopodia by backward peripheral con- 

 traction, as water can be forced out of a rubber tube by pressure from 

 behind. Others, again, imagined that in addition to the contractile 

 cortex the entire mass of the amoeboid body is penetrated by a con- 

 tractile substance (Cienkowsky, '63), the sarcous matter of Briicke 

 ('61). Still others conceived a contractile motor apparatus of even 

 greater complexity. Amongst these, Heitzmann ('73), in working out 

 his well-known theory of the structure of protoplasm, and adapting 

 Briicke's view to his own interpretation, maintained that the body of 

 Amczba is composed of contractile fibres and an inter-fibral " non- 

 contractile fluid." The protrusion of a pseudopodium, he argued, is 

 due to the local contraction or stretching of this fibrous framework. 

 Modifications of Heitzmann's view have frequently appeared in sub- 

 sequent writings. In connection with the Metazoa it still makes its 

 appearance in the numerous theories of contractile fibres, especially 

 in explanation of mitosis (van Beneden, Bovefi, Flemming, Reinke, 

 and many others). In connection with the Rhizopoda, it found its 

 most ardent advocate in R. Greeff ('91), who described radial, fibrillar, 

 contractile structures in the ectoplasm of many so-called Earth 

 Amcebce, and interpreted them as muscle-fibres whose outer ends 

 are inserted in the ectoplasm with their inner ends attached to the 

 protoplasmic framework of the endoplasm. Subsequent research has 

 shown that the supposed muscle-fibres are bacteria (Bourne, '91 ; 

 Israel, '94; Gould, '95). 



Contractility in a somewhat different form was also brought in to 

 explain pseudopodium formation. In connection with the Protozoa, 

 the most noteworthy advocate was Engelmann ('79), who conceived 

 units of contractile substance built up of molecules of protoplasm. 

 To these hypothetical units he gave the name inotogmata. During 



