GROWTH AND LONGEVITY 75 



Growth and Temperature of Environment.— The 



growth of poikilo therms and of young homoio therms is 

 limited by the temperature of the environment. A young 

 bird or mammal must be kept warm in a nest, marsupial 

 pouch, or uterus if development is to follow a normal 

 course. Yet such development does not necessarily need 

 to take place at temperatures Avhich are near those of 

 the bodies of the parents. An egg may mature in the 

 body of a hibernating frog and an embryo may continue 

 to develop in a hibernating mammal. After leaving the 

 body of the parent a frog's egg may rest in water be- 

 neath ice and develop normally as the temperature rises. 

 The eggs of various reptiles are commonly buried where 

 they are heated by the rays of the sun, as on warm hill- 

 sides, or where they may be heated by decaying organic 

 matter, as in rotting logs. A hen's egg may remain cool 

 for some time before development begins. It can later 

 endure for short periods temperatures which are below 

 the body temperature of the hen and still develop into a 

 normal chick. 



Homoiotherms in gaining the ability to maintain a 

 constant temperature through metabolic activity and 

 other means have also become able to provide a favor- 

 able temperature for the development of their offspring. 

 The eggs of frogs developing at 8 deg. C. require 30 days 

 to reach the same stage which they will reach in 8 days 

 at 21 deg. C. (Barthelemy and Bonnet, 1926). 



Determinate Growth. — Some animals grow less and 

 less as time goes on but appear to continue indefinitely, 

 others cease to grow when they reach maturity. In gen- 

 eral indeterminate growth is more characteristic of 

 poikilotherms than of homoiotherms. The prolonged and 

 more or less indeterminate growth which occurs among 

 fishes, amphibians, and reptiles is characteristic of 



