NATURAL HISTORY. 



protoplasm (with its innumerable floating molecules, granules, or globules), possibly a network of 

 filaments, and a more or less solid "nucleus"; and this last has often within it an almost 

 immeasurably small but distinct spot called the "nucleolus." Such "cells," being endowed with 

 vital force, can absorb and use up water and organic fluids ; they have the power of growth, of 

 secreting new materials, of producing similar " cells," capable of the same functions as those of the 

 parent " cell " (and even more advanced functions) ; and in many cases they can move freely. The 

 " nucleus," secreted or formed by the protoplasm, seems to regulate these vital phenomena, especially 

 germination or reproduction, for it multiplies itself by " fission," by breaking up into germinative 

 particles, and by the formation of " nucleoli," which, in their turn, become " nuclei." 



Even without a "cell-wall," the Amoeba is a true animal "cell": but the Protamceba, havinw 



7 O 



neither " cell-wall " nor " nucleus," represents 

 only the simple protoplasm of a " cell." Such 

 living corpuscles have been termed " Proto- 

 plasts" by some, and "Cytodes"* by others. 

 Such are the free-moving Monera (Prot- 

 amceba), the non-nucleated plasmodia of the 

 jlfyxomyceta, and the amceboid germs of 

 Gregarina, proceeding from the " pseudo- 

 naviculpe." That all these simple organisms, 

 however, are true animals has not yet been 

 satisfactorily determined. 



" The Amoeba, however," says Haeckel, 

 " presents the most simple form of a single- 

 celled ('unicellular') organism in a complete 

 state of development, and in some sort the 

 ideal of an animal ' cell.' Widely distri- 

 buted in fresh waters, on muds and wet 

 earth, and occurring in brackish and salt 

 water also, these animalcules are of special 

 interest on account of their eminently simple 

 structure as a ' cell,' and because of the 

 bearings of their development and functions 

 on the history and meaning of other ' cells.' " 

 V. Thus the Amcebce may be said to be 

 soft, naked, nucleated " cells," of indeterminable shape. They move here and there in water, 

 sometimes floating, but usually creeping on plants and other objects by protruding from any 

 part of the surface of their body, but more especially from one end, and that the broadest 

 and most translucent, variable finger-like lobes of their own body-substance, and then either 

 retracting these processes, called " pseudopods," or drawing the body to the point at which 

 they fix themselves. Of course the body varies indefinitely throughout these movements (see Figs. 

 1, 2, 3), being at one time nearly circular, at another angular, and then jutting out at corners or at 

 the sides with capes and peninsulas of no fixed shape, and ever slowly shifting, as if a floating 

 island, restless and bewitched, gained and lost its coasts again and again at the caprice of some 

 changeful sprite, aiming at fancied resemblances to hatids, antlers, or branches, and back again to 

 more solid but clumsy shapes of leaves and buds, and even slugs or imperfect stars.- 



" The changes of form produced by the extension and branching of certain of the pseudopods, 

 with the recession, melting away, and total disappeai'ance of others, is endless. Sometimes the 

 animal creeps onward in a flowing manner with comparatively simple cylindroid form, occasionally 

 emitting a single pseudopod on one side or the other. More commonly in movement it assumes a 

 dendroid or palmate appearance, or sometimes, diverging from the directly onward course, it becomes 

 more radiate. Not infrequently it assumes more or less grotesque shapes, in which almost 

 every conceivable likeness may be imagined." Leidy, " Freshwater Rhizop. N. America," 1879, p. 36. 



* Greek, cytos (plural cyta), a hollow ; eidos, appearance. 



Fig. 2. PKOTAMCEBA PKIMITIVA. 



Showing eomeof the stages during tho iiroooss of fission. Very highly 

 magnified. (After Hueckel'i 



