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. 
