Explanation of Ontogenetic Involutions 49 



The hypothesis of a continuous trophic nervous flux 

 being admitted, these serial unequal localizations of 

 growth can be explained by changes in the distribution of 

 this flux at each new ontogenetic stage, from causes 

 which we shall examine in the next chapter. These 

 changes of distribution bring about now here, now there, 

 a great affluence of nervous energy and thereby induce 

 at the corresponding points proliferation of the cells from 

 which must necessarily arise later the invagination or 

 evagination in question. 



But the ontogenetic phenomena which most clearly 

 call for the conception of such a distribution of nervous 

 energy which continually changes and shifts, streaming 

 now through one region now another of the developing 

 organism, are the phenomena of involution; that is to 

 say phenomena of reduction presented by the tissues of 

 an organ which after being formed in the course of 

 ontogeny tends at a later stage to disappear ; for example 

 the involution of the tail of a tadpole during its meta- 

 morphosis into a frog. 



The atrophy and degeneration of the skin, of the 

 notocord, of nerve and muscle fibers, by which this 

 involution is produced, have been described particularly 

 by Osborn. He as well as Metschnikoff has established 

 in this connection the great phagocytic activity of certain 

 cells and the formation of true and false giant cells. 

 Nevertheless he does not attribute to the phagocytes the 

 most important role in the elimination of material. The 

 whole of the process, in fact, results in the gathering 

 together of the cellular material in process of dis- 

 integration and conducting it into the lymph and blood 

 vessels for utilization later in the construction of other 

 organs and tissues peculiar to the adult animal. 



