286 C. M. CHILD. 



arrangement of the parenchyme cells probably has nothing to do 

 with active migration, but merely indicates the direction of the 

 strain which produces deformation. 



These histological features then, support and confirm the other 

 observations, and all appear to show that the change in shape is 

 primarily a deformation in consequence of strain rather than a 

 complex physiological process. 



There can be no doubt, however, as I have repeatedly pointed 

 out that reactions of various kinds result from the strain and de- 

 formation : muscles and other tissues undoubtedly " adapt " them- 

 selves to the new relations of parts. In fact there is no apparent 

 reason why the change of shape should not continue indefinitely, 

 or at least until the elasticity of the tissues became involved, if 

 nothing but the mechanical deformation took place. Undoubtedly 

 "functional adaptation " to the altered strains occurs and this 

 determines how far the change shall proceed. Sooner or later 

 the tissues adjust themselves fully to the new conditions, /. <?., a 

 condition of equilibrium is attained and change in shape ceases. 

 Under the usual conditions this gives what we commonly call the 

 " normal " shape, but I have shown above that under other con- 

 ditions the shape may be different. No one would deny, I sup- 

 pose, that the shape of cells is determined in many cases (e. g., 

 the polyhedral shape of blastomeres in many eggs, the polygonal 

 outline of many epithelial cells, etc.) to a greater or less extent 

 by purely mechanical factors. If this is the case it seems scarcely 

 possible to deny that purely mechanical factors may be concerned 

 in determining the shape and arrangement of parts in animals 

 without hard structures and with tissues of a high degree of 

 physical plasticity. That they are the only factors in such cases, 

 or that because they are factors in some cases, they must be in 

 others, I certainly do not and never have maintained. 



In Planaria the width of the head is apparently an important 

 factor in determining the width of the body behind it. The head 

 itself is not involved to any appreciable extent in the change of 

 shape. It furnishes, so to speak, a fixed point, or rather region 

 at one end, and between this and the posterior end the change of 

 shape occurs. Consequently, in short pieces, where the new 

 head is small, the change of shape is very much greater than in 



