^32 Comparative Anivtal Physiology 



like or may show local constriction rings according to the physical nature 

 of the medium."' The various forms of amoeboid cells are not always con- 

 stant for a species and one individual may take on different forms under 

 various conditions. Amoeba protens, for example, in distilled water becomes 

 stellate or radiate, whereas it may be active and monopodal in a dilute saline; 

 it is also stellate prior to fission. Amoebae have a central fluid plasmasol 

 and an outer viscous plasmagel. The thickness of the gel varies greatly 

 among different species: there is much gel in A. proteus, A. verrucosa, Pelo- 

 myxa carolinensis, and P. pahistris, but very thin gel in Amoeba duhia and 

 in the parasitic A. blattae. Amoeba dubia is useful, therefore, for measur- 

 ing by centrifugation the viscosity of the plasmasol, and A. proteus for meas- 

 uring viscosity of the plasmagel. ^^ 



GENERALIZED PICTURE OF AMOEBOID MOVEMENT 



There are many variations in amoeboid movement in the different cell 

 and pseudopod types, but a basic pattern is found in all. A "typical" amoeba 

 consists of a thin outer layer, the plasmalemma, which has adhesive proper- 

 ties, is not wet by water (is hydrophobic), and slides freely over the next 

 inner layer. Beneath the plasmalemma is a hyaline layer which is fluid as 

 judged by brownian activity when granules enter it. This layer is thin or 

 absent from a region of attachment to substrate; it often thickens as an ex- 

 tensive hyaline cap at the front of an advancing pseudopod. Next is the 

 shell or cylinder of plasmagel which is motionless and rather viscous. In a 

 monopodal amoeba the gel thickens gradually from the anterior to the pos- 

 terior end. In many species it is extended as a very thin plasmagel sheet 

 beneath the anterior hyaline cap, a sheet which frequently ruptures, allow- 

 ing granules to enter the hyaline cap. The core of the amoeba is the plas- 

 masol in which granules flow freely forward although the lateral margin 

 of it may be stationary. The nucleus is normally in the plasmasol, the con- 

 tractile vacuole in the plasmagel. Both regions contain various granules, 

 food vacuoles, and specific crystals contained in little vacuoles. The proto- 

 plasm is normally hypertonic (except possibly in marine and parasitic forms), 

 and the plasmagel imparts a turgidity which results in the non-spherical 

 cell forms. In species in which the sol of the pseudopods is continuous to the 

 tip (as in Difflugia') the plasmalemma must oppose free swelling. 



Locomotion depends on three basic factors, according to current views.^^ 

 The first is attachment to the substrate. This is facilitated by traces of salts, 

 particularly calcium, in the medium. Organic solutes such as lactose and 

 glycerin do not facilitate attachment; Ca, Mg, and K are additive for at- 

 tachment but not for locomotion. ■*-'' In amoebae, as in leucocytes, the firmest 

 attachment is at the tips, and new pseudopods are more firmly attached than 

 old ones. Second, plasmagel is continuously being converted to plasmasol 

 at the posterior end or at some fixed region, and plasmasol is converted to 

 plasmagel anteriorly or in any extending pseudopod. As plasmasol flows 

 forward granules either are deflected laterally to become plasmagel or break 

 into the hyaline cap and then gelate as a new cap is formed. Attachment 

 and reversible solation-gelation are readily observed. Third is the force that 

 causes forward flow of the plasmasol, which is less apparent. Most current 



