The Organism and Its Cells 173 



to two millimeters. . . . The activity of such fibers is noted 

 at the amoeboid end and consists in a constant retraction 

 and new formation of pseudopodia. All observations on 

 tin- movement of the growing fiber suggest an active force 

 within it causing jt s extension into the medium." 31 



Describing Hie activities still farther in connection with 

 the degeneration of some of the nerves, the author con- 

 tinues: "The change's in the nerves are mainly at the end. 

 Here there is periodic thickening, followed by a slow reduc- 

 tion in size until the entire nerve has retracted into the 

 h>Mie m a manner similar to the retraction of the pseu- 

 dopodia of an amoeba. These phenomena of extension and 

 retraction may go on alternately in the same fiber. . . . 

 The retraction is checked after a time and growth again 

 proceeds in a different direction for a while when the pro- 

 cess is again repeated." 32 



This primitive amoeboid activity of the cells is by far 

 the most common and readily accomplishable. The testi- 

 mony of all observers is at one on this point. But this 

 activity is by no means the only kind. Contraction of muscle 

 fibers was seen by Harrison, as mentioned in the quotation 

 already given ; a number of other investigators have con- 

 firmed and extended the observations. Burrows, for instance, 

 has shown that muscle cells from the embryonic chick heart 

 may contract rhythmically in cultures. He writes: "The 

 muscular elements grow much less frequently and cellular 

 outgrowths from them were observed in only about three 

 per cent, of the experiments. The outgrowths take place 

 from the myotomes and the heart, and appear in the form 

 of short chains of striated cells. The striated cells contract 

 rhythmically along with the portion of the heart from which 

 they arise." t0>t ' { 



Holmes has made the suggestive observation that although 

 completely isolated, partly differentiated muscle cells of a 

 newt may remain functionally active for eight months, 



