CHAPTER III 



SEGMENTATION, ORGANIC AND MECHANICAL 



Models may be and often have been devised imitating some 

 of the phenomena of division, but none of them have reproduced 

 the peculiarity which characterises divisions of living tissues, 

 that the position of chemical differentiation is determined by those 

 divisions. For example, models of segmentation, whether 

 radial or linear, may be made by the vibration of plates as in 

 the familiar Chladni figures of the physical laboratory, or by 

 the bowing of a tube dusted on the inside with lycopodium 

 powder, and in various other ways. The sand or the powder 

 will be heaped up in the nodes or regions of least movement, and 

 the patterns thus formed reproduce many of the geometrical 

 features of segmentation. But in the segmentations of living 

 things the nodes and internodes, once determined by the dividing 

 forces, would each become the seat of appropriate and distinct 

 chemical processes leading to the differentiation of the parts, 

 and the deposition of the bones, petals, spines, hairs, and other 

 organs in relation to the meristic ground-plan. The "ripples" of 

 meristic division not merely divide but differentiate, and when 

 a "ripple" forks the result is not merely a division but a re- 

 duplication of the organ through which the fork runs. An 

 example illustrating such a consequence is that of the half-ver- 

 tebrae of the Python. On the left side the vertebra is single 

 (Fig. 7) and bears a single rib, but on the right side a division 

 has occurred with the result that two half-vertebrae, each 

 bearing a rib, are formed, one standing in succession to the 

 other. We cannot, indeed, imagine any operation of physio- 

 logical division carried out in such an organ as a vertebra, passing 

 through a plane at right angles to the long axis of the body, which 

 does not necessarily involve the further process of reduplication. 



As the meristic system of distribution spreads through the 

 body, chemical differentiations follow in its track, with seg- 



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