410 PROF. TH. W. ENGELMANN. 



already rightly insisted, but must be applicable in its principle 

 to all other phenomena of contractility. The essential agree- 

 ment which exists between them all in the mode of appearance 

 and conditions of occurrence, and especially the gradual tran- 

 sition between them, show that we have here in all cases to do 

 with expressions of the same principle, with the same elemen- 

 tary mechanism of movement. As a starting-point for the 

 closer analysis of protoplasmic movement, we may take the 

 acknowledged fact that each smallest microscopically distin- 

 guishable particle of every contractile protoplasmic mass is 

 capable of independent movements. Ample proof of this is 

 furnished by the changes of form and position which may take 

 place, spontaneously or in consequence of artificial excitation, 

 at every point in any otherwise quiescent protoplasmic mass, as 

 also in the smallest artificially isolated protoplasmic particles. 



It follows as a very close and, 1 believe, most natural con- 

 sequence, that we may regard protoplasm as an aggregate of 

 most minute contractile, excitable form-elements, and that 

 the movement as a whole is the result of changes of form of 

 these very small elements. The nature and cause of the 

 changes of form of the latter remain provisionally undeter- 

 mined. 



We have, as yet, no reason for considering that the most 

 minute particles of protoplasm which are to be distinguished 

 with the microscope are the contractile elements them- 

 selves • we must think of the latter as still smaller — as of 

 molecular dimensions. With regard to their form, we may 

 take it for granted that when in a condition of maximal ex- 

 citation they are almost spherical, or as nearly spherical as 

 possible, and when not excited are generally elongated, with a 

 fibre-like shape. The reason for the first of these assumptions 

 lies in the observation that even the smallest particle of proto- 

 plasm which can be experimented upon assumes a spherical 

 shape as a result of artificial excitation, that is to say, if it be not 

 already spherical. Supporting the second assumption we have, 

 firstly, the fact that the smallest protoplasmic particles which 

 have contracted to spherical shape in consequence of excitation. 



