THE LOBOSA 69 



pseudopodia it can be clearly observed only on the pseudopodia 

 themselves (Fig. 12, 2). It seems probable, therefore, that in the 

 Lobosa there is no true differentiation of the cytoplasm, and that 

 the appearance known as ectoplasm is only due to the temporary 

 withdrawal of metaplasmic particles from the superficial parts of 

 the active cytoplasm. 



At the surface of an Amoeba there may always be seen a dark 

 border which has the appearance of a very thin pellicle. This 

 pellicle may be traced on the sides of the pseudopodia, but fades 

 away towards their extremities, becoming extremely attenuated at 

 the active terminal point. Immediately below this pellicle there 

 is a layer of very hyaline ectoplasm. In carefully prepared sections 

 the hyaline ectoplasm is found to be not strictly homogeneous, but 

 to possess an alveolar structure similar in general characters to that 

 of other forms of protoplasm. At the actual surface there is a 

 single layer of alveoli, in which, as in artificially prepared oil foams, 

 the sides vertical to the surface are parallel, or almost parallel, to 

 each other, giving the appearance of a row of fine vertical striae. It is 

 apparently this marginal alveolar layer which constitutes the pellicle. 



The movements of an Amoeba may be best interpreted on the 

 basis of the alveolar hypothesis of the structure of protoplasm. 



The protrusion of a pseudopodium begins with a lowering of 

 the surface tension of the marginal alveolar layer over a small area 

 on the surface. This is followed by a flow of endoplasm towards 

 the area of reduced surface tension. It has been suggested that 

 the initial stages are accompanied by a rupture of some of the 

 alveoli at the surface, which liberates a fluid the enchylema and 

 that this causes a local diminution of the surface tension. It is 

 possible that the release of enchylema may continue during the 

 whole of the process of the protrusion of a pseudopodium, and 

 stop when the pseudopodium comes to rest. During the active 

 protrusion of a pseudopodium there may be observed a rapid 

 centrifugal flow of endoplasm towards the peri- 

 phery, called the axial stream. At the apex this 

 stream spreads outwards like a fountain, and is 

 continued as return currents on the surface. Similar 

 fountain-like currents have been observed in the 

 movements of various artificially prepared foams, 

 but in the living protoplasmic pseudopodium the 

 velocity of the return currents diminishes more FIG. i. 



rapidly and soon comes to rest (Fig. 1). In an Diagram to show 

 Amoeba such as A. Umax, in which, as a rule, only 



one pseudopodium is formed, there is a reverse 

 fountain current at the posterior end, the particles at 

 the surface flowing towards the axis and joining in the axial stream 

 flowing in the direction of the advancing pseudopodium. But in 



