130 C. M. Child. 



often straightening will occur. This I believe is the chief factor in 

 the change from the curved to the straight bilaterally symmetrical 

 form. This piece died after sixty-six days without having become 

 completely symmetrical. 



The history of other pieces prepared in a similar manner Is 

 essentially the same, though, as has been mentioned, the degree of 

 curvature of the new tissue varies within certain limits with the 

 obliquity of the cut and with the level at which the cut is made. 

 The reason for variation in curvature with the angle of the cut 

 lies in the fact that the more oblique the cut the more the axis of 

 the piece is bent and consequently the greater is the curvature 

 of locomotion. 



As regards the level, the curvature of the new tissue decreases 

 as the distance of the cut from the anterior end increases, because 

 the region of the body in which the axis is not bent by the con- 

 traction of the cut surfaces increases and counteracts the asym- 

 metrical motor effect of the bent region more and more com- 

 pletely. This variation of curvature with the level of the cut 

 renders it evident that the cut surface itself has little influence 

 upon the direction of growth except in the earliest stages, for a 

 piece cut at a given angle near the anterior end, e. g. as in Figures 

 46-53, will give rise to new tissue with a marked curvature, while 

 in another specimen cut at the same angle but farther posteriorly 

 the new tissue will show much less curvature while in still another, 

 cut at the same angle in the posterior pharyngeal region, the 

 new tissue will grow out in the direction of the longitudinal axis 

 or will very soon acquire this direction, simply because the direc- 

 tion of locomotion is curved only very slightly or not at all. 

 In each case the direction of outgrowth coincides with the line 

 of locomotion, 



GENERAL CONSIDERATIONS. 



The bearing and the significance of the experiments described 

 is sufficiently clear to render extended discussion unnecessary. They 

 may, I think, be regarded as demonstrating the fact that the ex- 

 tended form of the body in Leptoplana is determined in large 

 degree by mechanical conditions. It is difficult to describe ac- 



