510 



Similar changes occui' in the brain ; but within it the deacl 

 larval elements lie for more than a day as masses of degenerate 

 cells, before these are absorbed by the imaginal cells. The 

 first ventral ganglion is merged int-o the brain, and it is much 

 more complex than in the larva. 



Leucocytes do not take part in the metamorphosis. 



J . The Vascida?' System. 



The blood acts as the medium into which the nutrient 

 degeneration products of the larva are poured and from which- 

 the imaginal cells again take them. Often the leucocytes, 

 which jDroliferate considerably during metamorphosis, may 

 aid in the removal of the dead larval elements by phagocy- 

 tising them. 



The larval heart consists of a long tube, provided with 

 ostia, and lying within a delicate pericardium. Imaginal 

 cells lie within the heart walls and regenerate it at the time 

 of defaecation. The larval pericardium undergoes total 

 degeneration. In its place is formed a band of cells right, 

 along the ventral side of the heart; the cells originate from 

 an imaginal disc lying ventral to the heart, just above the 

 end of the hindgut. This band of cells then grows upwards; 

 and completely surrounds the heart, forming a two-layered 

 organ, which becomes muscular behind (heart), remaining 

 non-contractile in front (aorta). Tliere is tlierefore no true 

 pericardium in the imago. 



K. The Insect Metamorphosis. 



Insect metamorphosis is brought about by an extensive 

 tissue death, due to the hypertrophy of the larval cells. 

 Death of the cells is due to an automatic starvation, due to 

 the fact that the cell contents, which increase as the cube 

 of the radius of the growing cells cannot be nourished 

 indefinitely through the cell membrane, whose area increases 

 only as the square of the radius. The hypertrophy must also 

 cause disorganization, as the distances through which diffu- 

 sions, etc., within the cells have to take place, become 

 appreciable. 



Metamorphosis has been evolved as an outcome of 

 specialization by the imago. A hard thick cuticle has 

 necessitated concentration of the growth period to the begin- 

 ning of the free living period, and the active life led by the 

 adults has made the carrying of large quantities of heavily 

 yolked eggs impossible. With continued decrease in the 

 quantity of yolk, the larva has had to ertierge at ever earlier 

 stages of its development. At metamorphosis we see the con- 

 tinuation of the interrupted development, and a recovery of 

 the organism from larval specialization. 



