86 FRESH-WATER ANIMALS 



herring is about the size of a pin's head, that of a 

 salmon is as big as a pea, while that of a dog-fish, 

 or of a hen is very much larger still. Every egg 

 contains two chief kinds of matter, germ-yolk and 

 food-yolk, the former of which develops directly 

 into the embryo, while the latter is simply a store 

 of food material generally in the form of minute 

 granules dispersed through the germ-yolk, which 

 can be drawn upon as required and at the expense 

 of which the embryo is able to develop. The 

 difference in size between one egg and another 

 concerns almost exclusively the amount of food- 

 yolk, which is very abundant in large eggs, but 

 comparatively scanty in small ones. Until the 

 food-yolk is used up there is no need for the 

 embryo to hatch : consequently embryos developed 

 from large eggs will be of much larger size and 

 greater strength at the time of hatching than those 

 developed from smaller eggs. This is well illus- 

 trated in the development of frogs. In the 

 ordinary frog the egg has but a comparatively 

 small amount of food-yolk and the embryo hatches 

 as a tadpole, an animal of small size and of much 

 simpler organisation than the frog. In the little 

 West Indian frog Hylodes however, the eggs are 

 of larger size i.e., they contain more food-yolk 

 and the embryo consequently hatches, not as a 

 tadpole, but as a fully-formed frog. 



As small free-swimming larvae are as a rule unable 

 to hold their own in fresh water it follows that 

 any increase in the size of egg i.e., in the amount 

 of food-yolk will be a direct advantage to a fresh- 



