kiNE TO GENESIS. 369 



would become the seat of the deposit. At the lines of 

 interruption joints would be formed, and if the move- 

 ments were habitually symmetrical, these interruptions 

 would be equidistant. In this way the well-known 

 segmentation of the external skeletons of Arthropoda, 

 and the internal skeletons of Vertebrata would be 

 formed. We have more detailed evidence that this 

 has been the case. Thus the segmentation of the os- 

 seous sheath of the chorda dorsalis in both primitive 

 fishes and batrachians has been accomplished in wedge- 

 shaped tracts precisely as may be observed in the fold- 

 ing of a tolerably stiff sleeve of a coat which ensheathes 

 the arm, under the influence of lateral flexures. The 

 wedge-shaped tracts are superior and inferior, the 

 apices directed towards each other. Seen from the 

 side they form two wedges with their apices together, 

 and their bases one up and the other down. Now, if 

 a person who wears a coat of rather thick material will 

 examine the folds of his sleeve as they are produced 

 on the inner side of his arm, he will see a figure nearly 

 like that of the segments of the vertebral column de- 

 scribed. The folds will correspond to the sutures, and 

 the interspaces to the bony segments. He will find 

 that the spaces are lens-shaped, or, when viewed in 

 profile, wedge-shaped, with the apices together. This 

 arrangement results from the necessary mechanics df 

 flexure to one side. In flexure of a cylinder like the 

 sleeve, or like a vertebral column, the shortest curve 

 is along the line of the greatest convexity of the cylin- 

 der. Here is the closest folding of the sheath, and 

 here, consequently, the lines of fold in soft material, 

 or interruption in hard material, will converge and 

 come together. That is just what they do in both the 



