THE PATH OF EVOLUTION 



giving rise thus to an envelope. It is only when a 

 congeries of these particles are grouped close together 

 that the appearance of a cellular structure arises. 

 The most striking characteristic of living protoplasm 

 is its temporary change of place, by a movement 

 of the mass as a whole, as well as the displacement, 

 and often the self-development, of its constituent parts. 

 The envelopes formed by the protoplasm from its 

 own substance take usually the form of tubular or 

 elongated cells, in which the slimy mass of protoplasm 

 is retained. This becomes filled with minute gran- 

 ules or corpuscles that are in constant motion,. prin- 

 cipally close to the sides of the cellules, leaving the 

 middle of the protoplasmic mass comparatively free. 

 In this latter there appear large vacant spaces or 

 cavities, the u vacuoles," in which a watery fluid, the 

 "sap juice," collects. The exterior of the vacuoles 

 becomes denser by the thickening of the protoplasm, 

 and which gradually form cross or diagonal bands in 

 the interior. Among these partitions the current of 

 protoplasm flows, carrying the corpuscles before de- 

 scribed, mostly in contact with the cellules sides. 

 The motion of the corpuscles and protoplasma is of a 

 twisting, boring character. They move along the 

 sides of the bands or cells continually down one 

 side, around and up the other. The particles move 

 more rapidly the smaller they are, the larger ones 



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