WOUND CLOSURE AND POLARITY IN METRIDIUM 469 



stump which has fonned a nipple is cut off, the nipple loosens 

 and the end which previously bore the nipple assumes the appear- 

 ance of the distal cut end of an excised .tentacle whose tip was cut 

 away after excision of the tentacle. 



When the animal is kept in chloretone solution until there is 

 no response to touch, muscle action being therefore eliminated, 

 the cut tentacle does not form a nipple. The initial inbending 

 occurs, and the opening grows gradually smaller by means of a 

 slow radial closure, which is completed in ten to twelve hours. 

 The nipple, therefore, results from muscular contraction. 



Polarity is seen to exist in Metridium tentacles, not only in the 

 well known fact that the effective stroke of the cilia is always 

 toward the tip, but also in the reaction to touch. This reaction 

 consists in contractions which are proximal to the point of con- 

 tact. Contraction of a narrow circular zone lying immediately 

 proximal of the point of contact is succeeded by a bend toward, 

 or in some conditions away from, the point of stimulation, and 

 this may be accompanied by a general contraction of the proximal 

 portion of the tentacle. Reactions in the excised tentacle are 

 similar but weaker. 



Polarity is shown also by differences in the wound-closing 

 reactions of the proximal and distal ends of excised tentacles. 

 In the distal cut ends the opening is temporarily closed or reduced 

 by a constriction comparable to, but not as pronounced as, that 

 which forms the nipple on an attached stump. The p'roximal 

 ends show no evidence of such a constriction. A rounded form 

 without wrinkles is characteristic of the distal end. The opening 

 of the proximal end is irregularly outlined and the walls are deeply 

 wrinkled longitudinally. Distal ends become structurally closed 

 and present a smooth, rounded surface. Proximal ends sometimes 

 remain open, but in other cases close. When they close, deep lon- 

 gitudinal wrinkling is characteristic. 



Tentacle fragments which were kept in chloretone in order to 

 eliminate the muscle activity close their distal ends by a slow 

 radial movement of the layers of the tentacle wall. The walls 

 of the proximal ends of such fragments are wrinkled longitudinally 

 and are strongly inroUed; they may or may not become closed. 



