497 



entirely to a growth in the size of the larval cells. Now as 

 these cells grow larger and larger there is formed an increasing 

 disproportion between the volume of the cell contents (which 

 increases as "the cube of the radius of the cells) and the sur- 

 face membrane through which the cell contents are being fed 

 (and which increases only as the square of the radius). 

 Eventually, therefore, a time must come at which the cell 

 contents cannot any longer receive sufficient nourishment 

 through the cell membrane. Death by starvation must be the 

 result: — indeed, the more rapidly the larva feeds, the sooner 

 it will starve. This great increase in the size of cells must 

 also have another effect. The delicate chemical reactions 

 occurring within the cells are of such a nature that they are 

 very efficient in minute cells; but it is quite conceivable that 

 as the distances through which diffusions and other molecular 

 movements, taking place within cells, become increasingly 

 larger, a critical volume will be reached at which the delicate 

 balance between the reaction is upset and cellular death is 

 the result. As the cells all grow approximately equally rapidly, 

 a simultaneous death of cells throughout the larva will occur ; 

 the leucocytes then fall upon the debris, and the result is 

 phagocytic histolysis. In insects which do not undergo meta- 

 morphosis, on the other hand, growth is produced largely by 

 cell proliferation. The cells do not reach their critical state 

 and extensive cell-death does not occur. 



While I am convinced that it is nothing but this great 

 cell growth to which the whole wonderful transformation is 

 to be attributed, yet I am aware that a number of objections 

 may be made against it. If, as it seems, the individual cells 

 have certain critical values beyond which life is no longer 

 possible, how is it that underfed larvae, in which the cells 

 have not reached this critical volume, may nevertheless meta- 

 morphose ? It is to be noted, however, that in underfeeding 

 (partially starving) the active larvae we are actively applying 

 the very factor which must inevitably appear as the cells 

 hypertrophy, viz., starvation. This objection may, indeed, be 

 turned into a strong support for the view which I have above 

 expressed. 



A more serious objection is that in cold weather the 

 larvae may live for months after the cells have attained the 

 critical volume. However, it has been pointed out by Chun 

 that the abundance of life in arctic and antarctic waters is 

 due to the greater length of life of the organisms living there, 

 due to a slackening of the metabolic processes with the lower- 

 ingf of the temperature. Probably the lowered temperature 

 acts similarly on these mature larvae, temporarily repressing 

 those chemical reactions within the cell which result in cellular 

 disorganisation . 



