60 C. M. CHILD. 



a considerable degree of elasticity appears justifiable. Under 

 normal conditions the body-wall is subjected to tension. By sec- 

 tion of the body at any point the internal pressure is removed 

 and collapse occurs ; the body-wall is no longer under tension 

 and contraction of the elastic layer begins. If the ectoderm and 

 muscles are to a large extent passive in this elastic contraction 

 the result will be not simply a reduction in surface area, but an 

 inrolling of the body-wall, since the mesoglcea is situated near 

 its inner surface. 



The fact that the region near the cut surface is always more 

 strongly rolled than other parts may perhaps be the result of the 

 direct injury to the tissues in this region, causing contraction, 

 but here as elsewhere the contraction must be greater in the 

 inner portions of the body-wall than in the outer, otherwise inroll- 

 ing could not occur. More probably, however, the greater 

 degree of inrolling near the cut surface is largely, if not wholly 

 due to the fact that the physical obstacles to the inrolling offered 

 by resistance of other tissues, etc., are much less near a free end 

 or cut surface than elsewhere and the effect of elasticity is there- 

 fore greater. It appears probable from the preceding consider- 

 ations that the mesoglcea plays the chief part in the inrolling 

 about the cut surface as well as in regions distant from it. 



Objection to this view may, however, be made on the ground 

 that a tonic muscular contraction resulting from the injury is not 

 only a possible but a much more probable cause of the inrolling. 

 A brief consideration of the facts is sufficient to show that the 

 inrolling cannot be explained as the result of muscular contraction. 



This is evident first from the fact that it occurs in all directions, 

 longitudinally and obliquely as well as transversely. If it were 

 the result of muscular contraction we should expect it to occur 

 only transversely since the body-wall contains only longitudinal 

 muscles. It is difficult to understand how the contraction of 

 longitudinal muscles could account for the inrolling of a longi- 

 tudinal cut margin, since the muscle fibers are parallel to the 

 cut. Moreover, there is no apparent reason why the inner por- 

 tions of the muscular layer should contract more strongly than 

 the outer, since all must be equally affected by the injury. And 

 finally, observation renders it very evident that the inrolling is 



