Mast, Light Reactions in Loiver Organisms. 145 



was found, in which the anterior end and a narrow portion of the 

 side extending nearly to the posterior end, were missing. This 

 segment oriented quite definitely. In swimming horizontally 

 toward a source of light it moved approximately parallel with the 

 rays, deflecting but little. When exposed to light from two sources 

 of equal intensity, it took a course about midway between them. 

 If the light from one of the sources was cut off after the segment 

 had thus oriented, it continued on its original course for a few 

 millimeters, then changed the direction of motion until it was 

 oriented once more. Its light reactions in general were like those of 

 intact colonies, but the path of this segment instead of bemg 

 straight as is true in case of entire colonies, was in the form of 

 a spiral. This was evidently the result of the mechanical effect 

 of the gap in the side ^nd rotation on the longitudinal axis. 



The reactions of many other segments of colonies were studied 

 later. Most of these segments were made by cuttingthe colonies in 

 pieces. In performing these operations a considerable number 

 were put under a cover glass which was then carefully pressed 

 down until the colonies split open. Under these conditions they 

 usually split at the posterior end, but sometimes at the side. By 

 inserting a needle ground to a knife-edge, the wall could be cut in 

 any direction desired without much difficulty. 



It was found that segments of practically all forms and sizes 

 responded to stimulation by light, but owing to their form and the 

 effect of gravitation, many could move only in small circles, and 

 were unable to orient. 



It can be stated definitely, however, that ?.mong segments of 

 various forms and sizes, such as are produced by cutting the colo- 

 nies in half, either parallel or perpendicular to the longitudinal 

 axis, respond in general like whole colonies, with the exception that 

 most of the segments take a spiral course, the width of which 

 depends upon the form of the segment. It is thus clear that a 

 colony of Volvox can orient when the anterior or the posterior end 

 or one side is missing. A theory of orientation must be broad 

 enough to explain not only the reactions of entire colonies but also 

 those of any segments. 



10. MECHANICS OF ORIENTATION. 



Jennings ('04, p. 32-62) found that Stentor coeruleus and 

 Euglena viridis orient by means of motor reactions when exposed 



