HABITS, ETC., OF OEREBRATULDS J-AOTELJS. 177 



ciliated epithelium given by Englemann (20) the fibrillas are 

 almost perfectly parallel^ and not only correspond exactly in 

 number with the cilia, but are just opposite to them. We 

 can hardly escape the conclusion that the external cilia are 

 prolongations of the internal fibrillae. If so, then the 

 difference between cilium and muscle-fibre is that the former 

 consists of a single fibrilla, while the latter is made up of 

 several bound together. 



Finally the fibrillas^, when they first appear, are not upon 

 the surface of either cell or spin-thread, but are distributed 

 throughout their substance. Hence they cannot be regarded 

 in any sense as '^surface secretions/' but are more inti- 

 mately associated with the cytoplasmic structure. If not 

 formed by a rearrangement of the reticular network, they at 

 least bear the same relation to the remainder of the cytoplasm 

 that such threads would. 



4. The first contractions of the changing pseu- 

 dopodia represent the combined action of all 

 cytoplasmic energy, pulling in a single definite 

 direction instead of in different directions. 



Originally the cytoplasm possesses perfect internal mobility, 

 and its first movements are amoeboid in character. Then 

 follow spinnings, or the putting out and withdrawal of pseu- 

 dopodia in different directions. Some of these become per- 

 manent, and the whole contractile energy of the cell is then 

 concentrated upon them. As the cytoplasm is pushed out 

 farther and farther it becomes more and more limited in 

 the direction of its movement. And when the pseudopodia 

 are once fastened and fibrillation has begun it can move only 

 in one direction, along the axis of the pseudopodia. 



The energy which is at first expended in different direc- 

 tions, some of them antagonistic to others, becomes concen- 

 trated along the axis of the pseudopodium. Hence the 

 resultant cytoplasmic contractions gain as much in strength 

 and rapidity as they lose in mobility, and become capable of 

 producing what we call a muscular contraction. The initial 

 contractions are noticeably weak, and the intervals between 



VOL. 43, PAKT 1. NEW SERIES. M 



