376 PROF. TH. W. ENGELMANN. 



which play a passive role with regard to the movement. Set- 

 ting aside as of casual occurrence solid particles which have 

 been taken in from without and nuclear structures, they — 

 namely, the granules and vacuoles contained in the protoplasm 

 — are generally of exceedingly minute size. The granules 

 may be very numerous, but on the other hand they may be of 

 very sparse occurrence. The majority of the granules are 

 albuminous, some are of a fatty nature while others are inor- 

 ganic (e.g. carbonate of lime, in certain Myxoplasmodia). 

 Occasionally coloured particles are present (many Myxomy- 

 cetes, Protamoeba aurantiaca, &c.). 



Very commonly the granules occur exclusively in the central 

 portions of the protoplasm. In this case a fairly thick glass- 

 like outer layer or skin devoid of granules may be distinguished 

 from a granular and therefore opaque central mass (this is 

 specially distinct in Amoebae and Myxoplasmodia). These two 

 may appear to be very sharply separated from one another 

 during actual movement, although they are continually be- 

 coming mixed and separated again. 



Where the granular protoplasm becomes drawn out into very 

 thin threads (pseudopodia of Rhizopods, Radiolaria, &c., the 

 thread-like networks of Noctiluca, numerous vegetable cells) 

 the granules often project beyond the superficial layer. Indeed, 

 in such cases they often exist chiefly in the superficial layer. 

 Foreign particles moreover easily get stuck to the outer layers 

 of naked protoplasm, and are then moved along in the same 

 way as the true granules (Rhizopods, Oscillatoria, Diatoms, 



&c.). 



The densely granular portions of the protoplasm appear as a 

 rule to possess less cohesion than those which are devoid of 

 granules. The granular ceittral mass of Myxoplasmodia and 

 Amoebae often flows within the firmer superficial layer like a 

 fluid emulsion in a bladder. Not unfrequently the granules 

 exhibit irregular shaking, dancing movements, apparently quite 

 similar to those exhibited by the smallest particles suspended 

 in a thin fluid (Brownian molecular movement). This is the 

 case for instance in t^ie eudoplasm of Vorticellae, in the interior 



