What is to be said of the other leading trait which the simplest vertebrate animal has in common with all higher vertebrate animals--the segmentation of its lateral muscular masses? Is this, too, explicable on the mechanical hypothesis? Have we, in the alternating 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 surface 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, entailed by alternating 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 modifications progress? Every one has literally at hand an example of the way in which a flexible external layer that is now extended 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 gradations 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 thin, but more decided and comparatively distant fissures as they become thick. Fig. 289, which is a horizontal longitudinal section, shows how these thickening layers will adjust themselves on the convex and the concave surfaces, supposing the fibres of which they are composed to be oblique, as their function requires; and it is not difficult to see that when once definite divisions have been established, they will advance inwards as the layers develop; and will so produce a series of muscular bundles. Here then we have something like the _myocommata_ [or myotomes as now called] which are traceable in the _Amphioxus_, and are conspicuous in all superior fishes.