499 



attainment of a critical cell volume — niay bring about the 

 sudden transformation. In these forme divergence of the cells 

 from the imaginal condition has not proceded so far as to 

 prevent the cells returning to it; but in the more profound 

 metamorphoses the greater specialization of the larval cells 

 has resulted in a far more marked departure from the imaginal 

 type; the cells are unable to recover when they reach the 

 critical volume, and death is the result. 



Cell rejuvenation, then, is to be looked upon as a sudden 

 differentiation in a cell in which this has been, for some 

 reason, delayed; it differs from differentiation as more usually 

 seen, in that here the process is gradual. No satisfactory 

 explanation has yet been offered for the extraordinary 

 phenomenon of cell differentiation — ^the gradual transforming 

 of a cell from a non-differentiated truly embryonic state into 

 one whose structure is correlated with its function. But the 

 "abnormal" differentiations to be observed in metamorphosing 

 insects seem to throw considerable light on the process. It 

 is well known that non-differentiated cells may quite success- 

 fully perform work which is usually carried out by differenti- 

 ated cells. For instance, the heart of a chick embryo beats 

 long before cardiac muscle becomes .differentiated. I have 

 similarly observed undifferentiated muscle cells of Nasonia 

 functioning successfully. Now, it is well known that cell 

 growth and cell differentiation are parallel events in embryonic 

 processes. Tissues consisting of cells with definite hereditary 

 characteristics are laid down; the constituent cells grow and 

 the tissues, or better, their component cells, differentiate. 

 Is it not possible that in the struggle for existence that must 

 ensue as the cells grow in size, all but the non-essential sub- 

 stances — all those substances to which the generalized con- 

 dition of the embryonic cell is due — gradually disappear? 

 Only those substances which are essential persist. The 

 "explanation" is very incomplete, and there are many diffi- 

 culties in the way of its acceptance. But it seems to me that 

 it contains an element of truth. 



It is necessary to consider next the course of evolution 

 of the insect metamorphosis, and to consider the factors 

 which have necessitated this evolution. Lubbock has pointed 

 out that a metamorphosis is a necessity in organisms whose 

 adult and larval mouth parts are structurally unlike. This 

 does not, however, explain the reason for the metamorphosis 

 of the more insignificant structures of the insect's body. Nor 

 does it help us to understand why the insect larva which shows 

 this metamorphosis should ever have been evolved. 



The phylogenetic significance of the larva has, I believe, 

 frequently been explained correctly. It is a stage which has 



