511 - G. 0. UOURNE. 



altogether. This process, which is of the greatest importance, 

 is called hy Kleinenberg the development of organs by substi- 

 tution. The organs resulting from the changes are the organs 

 of substitution ; those through which the changes are effected 

 are the intermediary organs. In no case of substitution are 

 the intermediate steps represented by an indifferent germ layer, 

 but always by a functional and specifically differentiated organ. 



One of the most striking instances of substitution is the 

 development of the axial skeleton of the Chordata. Both phy- 

 logenetically and ontogenetically the transformation of an 

 already existent specialised portion of the gut leads to the 

 formation of a supporting skeletal rod, — the notochord. When 

 once this is established it induces correlative changes in the 

 muscles, which act more efficiently because of the firmer sup- 

 port. The muscular changes react upon the supporting rod, 

 which no longer affords a suitable point d'appui for them, 

 and is replaced by, not converted into, a segmented cartilagi- 

 nous column. The very segmentation is caused by the muscles, 

 for the cartilage is primitively unsegmented. Finally, the 

 cartilage is inefficient, and is replaced by bone. It is most 

 important to observe that each organ is functional and com- 

 plete in itself for a time, and does not form any part of the 

 organ which subsequently replaces it, but is entirely or very 

 nearly aborted. The disappearance of the notochord is an 

 excellent example, but many others could be given, all equally 

 good — for instance, the successive development of pronephros, 

 mesonephros, and metanephros. 



The author claims that the theory of the development of 

 organs for substitution gives an entirely new significance to the 

 persistance of rudimentary organs. That organs which have had 

 no functional importance should persist through long series of 

 generations is surprising and inexplicable by conventional views. 

 But if once it is recognised that they are intermediary organs 

 their use in ontogeny becomes apparent. Phylogenetically 

 they induced the formation of the organs which replaced them, 

 therefore they are retained in order that, in ontogeny, they may 

 give the impulse to the formation of those new organs of sub- 



