STRUCTURAL DIFFERENTIATIONS 133 



dium of Amoeba proteus is certainly a less complex motile organ 

 than the flagellum of Euc/Iciia ciridis, and therefore more primitive. 

 Had the comparison been made between the pseudopodia of Adino- 

 phrys sol or Acanthocystis acvleata and a typical flagellum, the con- 

 clusion would not have been so obvious. There is a good deal of 

 evidence against the generalization as it is usually expressed. In 

 the first place, a pseudopodium of Amoeba proteus cannot be inter- 

 preted as a motile organ. It is not a definite structure in the cell, 

 not does it cause the body of Amoeba proteus to move. On the con- 

 trary, it exists because of the movement of the body protoplasm 

 and the pseudopodium is merely the visible, physical expression of 

 this movement which, in turn, is due to the transformation of energy 

 in destructive metabolism. This energy finds its vent in that por- 

 tion of the ectoplasm which, for the time being offers the least resist- 

 ance; the ectoplasm gives way at this point, the endoplasm gushes 

 through and a pseudopodium results (see Chapter IV, p. 172). 

 Such pseudopodia are not the source of movements of the cell, 

 they are results, not causes, of movement. The pseudopodia of 

 Heliozoa, on the other hand, are motile organs, and the axial fila- 

 ments which they contain are regarded as equivalent in struc- 

 ture and in mode of origin to the kinetic elements of flagella. The 

 pseudopodia of Foraminifera are intermediate between those of 

 Heliozoa and those of testate rhizopods. The problem, then, comes 

 down to a theoretical question of probabilities. Is it more probable 

 that pseudopodia of the type found in Amoeba proteus become pro- 

 gressively difl'erentiated into motile organs through stages like the 

 finger-formed pseudopodia of the testate rhizopods, the reticulate 

 pseudopodia of Foraminifera and axopodia of Heliozoa and Radio- 

 laria, to the topical motile organ of the flagellate type? Or is it 

 more probable that a motile organ originating from a definite kinetic 

 center (basal body or blepharoplast) has become progressively indefi- 

 nite with loss of the kinetic elements through the same series of 

 forms, but in the opposite direction, and ending in types like Amoeba 

 proteusf To my mind, the pseudopodia of Amoeba proteus and its 

 immediate relations, have no place at all in such a series; they are 

 merely expressions of the ph^•sical conditions of the protoplasm and 

 of the forces operating within, and they may appear in any cell 

 having an appropriate physical make-up. Thus we find them in 

 certain types of cell (leukocytes and phagocytes) widely distributed 

 throughout the animal kingdom, and we find them here and there, 

 in every group of the Protozoa. 



An illuminating illustration in support of this conclusion is 

 afforded by the transitor\- flagellated stages of one group of amoe- 

 boid organisms, the Bistadiidae (see p. 337). Here, in Ndgkria 

 gruberi, for example, the organism loses its pseudopodia under cer- 

 tain conditions, and develops flagella, not b}' metamorphosis of the 



