498 



The extensive tissue disruption which occurs at meta- 

 morphosis is due, then, to the hypertrophied state of the 

 cell, this in turn being the result of a failure of the larval 

 cells to divide. A more complete explanation ought, then, 

 to account also for this absence of cell division with the devel- 

 opment of specialization by the larval cells. Now I have 

 pointed out, in the foregoing account of the metamorphosis of 

 Nasonia, that the larval nucleus does not seem to be able to 

 increase its chromatic contents. A volumetric* increase — 

 sometimes quite considerable — of the nucleus is frequently 

 seen, but no increase in the quantity of chromatin is ever to 

 be detected. There may be an increase in the number of 

 chromatin granules, but these are always formed by a break- 

 ing up of the karyosome of the nucleus of the young cell 

 (fig. 102). Now it has been pointed out by Professor Brails- 

 ford Robertson (1909) that cholin formed as a by-product 

 during the synthesis of nuclein from lecithin, accumulating 

 mainly at the median plane of the cell, would bring about a 

 diminution of surface tension here, and division would result. 

 In the growing larval cells such nuclein synthesis is apparently 

 absent, and consequently no cell division is to be expect-ed. 

 Specialization may perhaps consist then, in part, simply in 

 the loss of the nuclein synthesizing enzymes. 



The investigations of Poyarkoff have shown that in 

 Galeruca many of the larval cells transform themselves into 

 imaginal cells by undergoing a process of rejuvenation; por- 

 tions of the old cells are cast out and the interactions of the 

 remaining substances transform the cell into that of the adult. 

 The cytological interpretation of these results is very diflficult. 

 But is it not possible that those cell substances which have 

 (phylogenetically) recently been acquired — substances the 

 acquisition of which has enabled the cells gradually to adapt 

 themselves to the new conditions, as the feeding habits of the 

 evolving larva gradually diverged from those of the adult — 

 may become starved out from the cells as they gradually 

 approach the critical volume? These substances are, in a 

 sense, "foreign" to the cell; while the cell lived under its 

 new environment (the larval environment) they thrived ; but 

 as the cells gradually weakened with increasing cell volume, 

 would it not be these very substances which would perish first ? 

 And is it not possible that the substances which Poyarkoff 

 observed emerging from the rejuvenating cells were nothing 

 but the substances to which the larval cells owe their new 

 properties? These considerations will become clearer when 

 we have examined the phylogeny of the insect metamorphosis. 

 It is sufficient to point out here that even in the "cell 

 rejuvenation" type of metamorphosis the same stimulus — the 



