EMBRYOLOGY OF (RYPTOBRAXCHUS 539 



5. Critical periods in the life history 



111 closing this section it remains to note that there are three 

 stages in the hfe history characterized by unusual mortality — 

 nature's examinations occur in these stages. These critical 

 periods of development are: 



Fig. 202 Spiral -tailed monster of Cryptobranchu.s allegheniensis two months 

 after hatching. Photographed from the living larva anaesthetized with ehlore- 

 tone. X 3. 



(a) The late hlastula. Many eggs seem unable to form a gas- 

 trula, 



(6) The hatching period. Many embryos seem to lack the 

 strength necessary to escape from the gelatinuous envelope, 

 and die in the capsule. 



(c) The period of change in the method of nutrition. After ex- 

 hausting their supply of yolk, many larvae refuse to take food, 

 or seem unable to set up digestive processes, and die of starvation. 



