THE PHYSIOLOGY OF EXCRETION. 93 



know is freed by the above-mentioned soaping process. This 

 stimulus acts upon the flagellum, which transmits it to the 

 middle piece. The middle piece transmits it,, changed into 

 impulse, to the basal rod, which contracts and expands rapidly 

 and thereby reacts upon the middle piece, which, being in con- 

 tact with the neighboring middle pieces, distributes this dis- 

 turbance over the whole row of middle pieces. I am opposed 

 to the general view that the flagella are contractile, because 

 we never could get any vibratory motion as a result of their 

 contraction, and, moreover, we ought to observe an elongation 

 and shortening of the flagella which has never yet been 

 observed. 



If, on the other hand, we accept a rapidly alternating con- 

 traction and expansion of the basal rods, the ciliary motion is 

 easily explained. The contraction of the rod drags the flagel- 

 lum a short distance toward the cell surface, and during the 

 following expansion the flagellum, being elastic, will be bent on 

 account of the resistance of the medium. The fact that the 

 rods are all slightly inclined towards the cell surface explains 

 how the flagella make a stronger inclination to one side. The 

 continuous wave of ciliary motion over a whole ciliated surface 

 is explained by the continuous contact of the middle pieces. 



If my theory is true we have here a minute nervous system 

 in one cell: (i) The flagellum, the receiver and conductor of 

 stimuli. (2) The middle piece, a motor centre which transmits 

 the stimuli, changed to impulses, to (3) the contractile rods, 

 which represent the muscular system. The rod contracts and 

 returns a sensational impulse to the middle piece, which dis- 

 tributes it peripherally to the neighboring middle pieces. The 

 middle piece acts here as a centre for the transmission of 

 impulses, both centripetal and centrifugal, which is an ex- 

 tremely simplified mechanism for nervous transmission. 



I assume that the stimulus comes from the outside because 

 if it originated in the cell all the rods should contract at the 

 same time and no continuous ciliary wave could result. 



This extreme activity means constant use and loss of mate- 

 rial and energy. In order to be active an organ has to be well 

 fed and supplied with a certain amount of free oxygen. The 



