244 Journal of Comparative Neurology. 



any rate none could be detected with the means of observation 

 and measurement employed by him. The animal now seemed 

 to have no appreciation of the vertical or recognition of its posi- 

 tion relative thereto. In locomotion likewise the animals were 

 uncertain and timid after these operations. As a result of all 

 his experiments Clark believes that the otocyst without the oto- 

 lith is able to act as an equilibrium organ. 



Although Bethe has argued that the stone is a necessary 

 part of an equilibrium organ, Clark's view would seem at least as 

 defensible in theory as it is demonstrable in fact. Even though 

 no stone or body of great density be present, it is true that 

 with every change of orientation with respect to the vertical, 

 new stress relations between the parts of an organ must be set 

 up. These would be especially strong in hairs projecting into 

 a cavity. In one position of the body it might be that the 

 weight of a hair would press perpendicularly upon its base. In 

 another position of the body, the same hair would pull upon 

 its base. Or it might be bent to one side. It is true that this 

 pull or bend would be small, especially is a cyst filled with 

 liquid. But we know that a very small stimulus is sufficient in 

 these organs to call forth a response. Nor do we need to im- 

 agine a movement of the hair as a whole. A mere change in 

 the stress of its cells or of parts of the cells might act as a 

 stimulus. Such a supposition indeed would go far toward har- 

 monizing the diverse phenomena of geotropism. Many infu- 

 sorians have a definite orientation in the water. It is known 

 furthermore in many cases that different parts of the cells, for 

 instance cytoplasm and nucleus, have different densities. It is 

 imaginable that the pull of the nucleus in one direction or its 

 pressure in the other might render the ciliary action on one side 

 different from that on the other ; that only in one orientation, 

 (for instance in Paramoecium, with anterior end directed upward) 

 would the action of the cilia on opposite sides of the body be 

 equal. In plants too, geotropic phenomena must be traced 

 back to the cells. I have mentioned the nucleus and cytoplasm 

 only as parts whose stress relations may possibly be changed by 

 gravity. The large and complex molecules of living matter 



