A Community of Cells 79 



the active substance is removed by the sequence of 'light' reactions. 

 Growth-promoting substances or hormones are also found in animals 

 (see next Chapter). 



This account of development may be concluded by mentioning 

 the extraordinary transformations which insects, etc, undergo in 

 their hfe cycles. The larva which hatches out from the egg may be an 

 entirely different creature from the final insect, different in form, 

 habits, food and construction. A butterfly or moth comes out of the 

 egg as a gaily coloured caterpillar, which, as it eats and grows, sheds 

 its hard skin several times. When it is fully grown, it winds itself into 

 a cocoon and goes into a dormant state. Its tissues are dissolved and 

 refashioned; and it ultimately emerges as an entirely different 

 creature, the finished butterfly, which can lay eggs from which new 

 larvae emerge. The genetic material carried by the chromosomes re- 

 tains its integrity throughout these changes. 



The moulting stages and the final metamorphosis into the adult 

 insect have been carefully studied by Dr V. B. Wigglesworth in a 

 blood-sucking bug, Rhodnius prolixus. This goes through five stages 

 as a larva, with a moult between each, and then metamorphosis into 

 the complete adult occurs. Between each moult it needs a single large 

 meal of blood, and this starts off the train of events leading to the 

 moulting, which takes place 12-15 days later. It has been shown that 

 the moulting process is caused by a hormone, which is secreted into 

 the blood by a gland situated in the brain. 



It has also been found that the premature change into the adult is 

 prevented by another hormone (the juvenile hormone) which is pro- 

 duced by the corpus allatum, a gland just behind the brain. If these 

 glands are removed by chopping off the head of the larva at an appro- 

 priate interval after feeding, metamorphosis into the adult form takes 

 place. On the other hand, if the head of a young larva is implanted 

 into the fifth stage larva, which is on the point of turning into the 

 adult insect, the metamorphosis is suppressed and a giant sixth stage 

 larva is formed instead of an adult. ^ 



