74 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



form of an irregular net-work, as represented in Fig. 46. These proto- 

 plasmic threads are the seat of changes, both of form and position. The 

 single filaments may become thicker or thinner, or a new filament may 

 spring out from and enter and unite with adjoining filaments, or may 

 undergo division into several others, the process being analogous to that 

 already described as characterizing the amoeba. In addition to this 

 motion in the cell-contents, rotatory movements may also be seen to take 



place in the protoplasm which is in 

 contact with the walls of the cell, 

 rotation occurring in a constant 

 direction and with almost uniform 

 rapidity around the cell-nucleus, the 

 imbedded chlorophyll and starch- 

 granules rotating in a mass without 

 airy decisive change in their relative 

 positions. Such rotatory move- 

 ments are seen in the leaf-cells of 

 the Yallisneria, and various other 

 plants. Similar motions are also 

 seen in the paramoecium and other 

 infusoria. 



In young animal cells the same 

 character of movement is often 

 present ; often when a membrane is 

 absent or is very flexible the pro- 

 toplasmic movements cause a change 

 in the entire shape of the cell, and 

 the motion so produced cannot be 

 distinguished from those of free 

 protoplasm. 



Occasionally protoplasm be- 

 comes free by escaping from the 

 interior of cells, such as the so-called 

 plasmoids of myxonrycetes, in which 

 not only an internal granular move- 

 ment but also a change of external 

 shape may be made out. Similar phenomena are also seen in those 

 organisms which consist of masses of free protoplasm, such as the 

 monera, rhizopods, polyps, and infusoria. Such protoplasm possesses 

 in an eminent degree the property of contractility,— a term originally 

 applied to striped and unstriped muscles. The changes in form of 

 masses of free protoplasm is identical in nature with that observed in 

 muscular contraction. 



Fig. 46.— Tradescantia Cells, after 



KtJHNE. 



A represents the fresh cell suspended in water; B the 

 same cell after moderate, local electrical stimulation. 

 The irritated region extends from a to b, the protoplasm 

 being collected into clumps, at c and d, as a result of the 

 stimulation. 



