PROTOPLASMIC MOVEMENT. 375 



refractive index than water but lower than oil. In some cases^ 

 as, for instance, where it has the form of thick fibres or skin- 

 like layers with a prevailing movement in one direction (pseu- 

 dopodiaof Actinosphoerium Eichornii, cortical protoplasm 

 of Stentor), it is distinctly doubly refracting, and, indeed, as 

 in the case of muscles and cilia, with a single positive axis, the 

 optical axis coinciding with the direction of the movement.^ 



Different portions of one and the same protoplasmic mass 

 may have different refractive powers. In the case of naked 

 amoeboid protoplasm the more superficial layers are more 

 highly refractive than the deeper: in tlie pseudopodia of Acti- 

 nosphoerium and many Rhizopods a strongly refracting axial 

 layer may be distinguished from the remainder. During the 

 movements the refractive power of the same portion usually 

 changes to a considerable extent. 



With regard to its mechanical properties, protoplasm may 

 present a greater or less degree of fluidity, does not mix with 

 water, and is capable of swelling up ; it presents great cohesive 

 power, great extensile power, trifling elasticity and a tendency 

 to take on the form of droplets. These properties vary, how- 

 ever, not only for different varieties of protoplasm, but at 

 different spots of the same protoplasmic mass, and often differ 

 even in the same spots within short intervals of time. With 

 naked ama'boid protoplasm the superficial layer is firmer than 

 the central mass and may even permanently or temporarily 

 pass into a strong membrane. As a general rule, no such 

 membrane exists so that solid particles can be taken in by the 

 outer layer of the body at any chosen spot, as may be easily 

 observed by feeding with coloured particles (indigo, carmine, 

 &c.).^ In many cases the central mass is the firmer, the 

 superficial portion being very soft and often quite sticky (pseu- 

 dopodia of many Rhizopods, Actinosphoerium, &c.). 



Protoplasm almost without exception contains certain bodies 



1 ' Arch. f. d. Ges. Physiol.,' xi, p. 449 and 454, &c., 1875. 



^ E. Haeckel, ' Die Radiolarien,' pp. 104 — 106, v. Recklinghausen, ' Arch. 

 f. pathol, Anat.,' xxviii, p. 184; W. Preyer, ibid, xxx, p. 420; M. Schultze, 

 •Arch. f. mikroscop. Anat.,' i, p. 23. 



