198 MORPHOLOGICAL DEVELOPMENT. 



cular masses ? Is this, too, explicable on the mechanical 

 hypothesis ? Have we, in the perpetual transverse strains, 

 a cause for the fact that while the rudimentary vertebrate 

 axis is without any divisions, there are definite divisions 

 of the substance forming the animal's sides ? I think we 

 have. A glance at the distribution of forces under the 

 transverse strain, as represented in the foregoing diagrams, 

 will show how much more severe is the strain on the outer 

 parts than on the inner parts ; and how, consequently, any 

 modifications of structure eventually necessitated, will arise 

 peripherally before they arise centrally. The perception of 

 this may be enforced by a simple experiment. Take a stick 

 of sealing-wax and warm it slowly and moderately before 

 the fire, so as to give it a little flexibility. Then bend it 

 gently until it is curved into a semi-circle. On the convex 

 surface small cracks will be seen, and on the concave sur- 

 face wrinkles ; while between the two the substance remains 

 undistorted. If the bend be reversed and re-reversed, time 

 after time, these cracks and wrinkles will become fissures 

 which gradually deepen. But now, if changes of this class, en- 

 tailed by perpetual transverse strains, commence superficially, 

 as they manifestly must ; there arise the further questions — ■ 

 What will be the special modifications produced under these 

 special conditions? and through what stages will these modifi- 

 cations progress ? Every one has literally at hand an example 

 of the way in which a flexible external layer that is now ex- 

 tended and now compressed, by the bending of the mass it 

 covers, becomes creased ; and a glance at the palms and the 

 fingers will show that the creases are near one another 

 where the skin is thin, and far apart where the skin is thick. 

 Between this familiar case and the case of the rhinoceros- 

 hide, in which there are but a few large folds, various grada- 

 tions may be traced. Now the like must happen with the 

 increasing layers of contractile fibres forming the sides of 

 the muscular tunic in such a type as that supposed. The 

 bendings will produce in them small wrinkles while they are 



