112 EMBRYOLOGY 



3° C. to 24° C. Above and 'below these temperatures development is 

 abnormal. 



Other environmental factors change the course of development. The frog 

 egg must be raised in a weak salt solution. Strong salt solution, such as 

 Ringer's solution, always causes early death of the eggs. Thus, although 

 Ringer's solution is a suitable medium for the tissues of the adult frog, it 

 is toxic to the developing egg. 



An external supply of food and various salts is not necessary for develop- 

 ment until some time after the larva hatches from the egg membranes and 

 jelly. The yolk of the unfertilized egg contains all the substances necessary 

 to supply the developing egg well beyond the hatching stage. 



The volume of water in which the egg develops is important. The egg 

 uses oxygen continuously and excretes carbon dioxide, ammonia, and urea. 

 If the eggs are crowded together in a small volume of water they die as a 

 result of a combined lack of oxygen and accumulation of waste products. 



Breeding habits and fertilization 



In nature the eggs of Rana sylvatica are laid in masses containing about 

 2000 eggs each and appear quite crowded. However, the temperature of the 

 environment at the season when ovulation takes place is relatively low, and 

 therefore the rate of utilization of oxygen and the rates of excretion of 

 carbon dioxide, ammonia, and urea are also relatively low. Thus development 

 is possible even under crowded conditions. If eggs from a natural pond are 

 observed carefully it is seen that some of them are retarded in development 

 and some die. This normal mortality is not necessary in the laboratory, 

 however, since the eggs may be given more space for development and 

 the mortality consequently greatly reduced. The large number of eggs laid by 

 each female is necessary, since large numbers die before reaching maturity. 



Other species of frogs have different breeding habits. Some frogs lay their 

 eggs in thin, flat sheets and thus crowding is somewhat reduced. Some toads 

 lay their eggs in a long chain in single file, and some salamanders deposit 

 eggs one by one, wrapping each egg with the leaves of a water plant. It is 

 interesting that in this latter case only a few eggs are produced by each adult 

 salamander. The chances of survival of the egg are so favorable, owing in 

 part to the breeding habits of the adult, that large numbers of eggs are 

 unnecessary. 



