178 PROTOZOA. 



finger-like prolongations, which sometimes branch, and, after a time, 

 are again drawn in. With this change of form there has also been a 

 change of position, the cell creeps along by means of its processes, 

 moving slowly, but in a definite direction. Solid particles are thus 

 enclosed by the soft protoplasmic mass, and, if their nature admit of 

 it, may be dissolved or altered. We can even feed the cells in the 

 same manner as Infusoria with particles of .pigment, and watch these 

 as they remain for a while within the body substance. 



It is not difficult to understand the lively interest which these 

 phenomena excited when first closely observed in the white blood- 

 corpuscles of vertebrates. 1 And this was naturally intensified when 

 men became convinced that they had here to do not with a unique 

 peculiarity of these corpuscles, but with a fundamental property of 

 animal protoplasm. Of course some cells may exhibit this pheno- 

 menon in a particularly conspicuous and lively fashion. Thus, only to 

 mention one or two examples, the blood-cells of lower animals (e.g., of 

 Thetis, Fig. 93), according to Haeckel, may be much more easily fed, 

 and exhibit more conspicuous movement, than those of vertebrates, 

 which, for the purposes of observation, require the aid of a warm stage 



FIG. 93. Blood-corpuscles of Thetis, partly with enclosed granules 

 of pigment (after Haeckel). 



or similar appliance. Again, the membraneless eggs of sponges and 

 polyps exhibit amoeboid movements in a most wonderful way. The 

 same is true of segmentation spheres, which often show beautiful 

 " migrating cells," which not unfrequently devour and digest the 

 granules of the yoke, just like independent organisms. 2 



This comparison is all the more necessary since there are numer- 



1 Max Schultze, Archivf. mikrosk. Anat. t Th. i., p. 1 et seq., 1862. Among previous 

 observations I may specially mention those of Lieberkiihn. Mutter's Arckir f. Anat. u. 

 Physiol., p. 14, 1854. 



* See Reichenbach, " Die Embryonalage und erste Entwicklung des Flusskrebses," 

 Zeitschr. f. wiss. ZooL, Bd. xxix., p. 153, 1877 [and also Metschnikoff, Quart. Jmirn. 

 Micr. Sci.< voL xxxiv., p. 89, 1884. W. E. H.]. 



