224 



EVOLUTION AND ANIMAL LIFE 



or in the egg stage, as it is variously termed. The hfe of an 

 animal as a distinct organism begins in an egg. And the true 

 life cycle of an organism is its life from egg through birth, growth 

 and development and maturity to the time it produces new 

 organisms in the condition of eggs. The life cycle is from 

 egg to egg. Birth and growth, two of the phenomena readily 

 apparent to us in the life of every animal, are two phenomena 



Fig. 131. — Leptodera hyalina, showing sex dimorphism: A, Head of male; B, head of 



female. 



in the true life cycle. Death is a third inevitable phenomenon 

 in the life of each individual, but it is not a part of the cycle; 

 it is something outside. 



The single cell formed by the fusion of two germ cells is 

 called a fertilized egg cell, and its subsecjuent development 

 results in the formation of a new individual of the same species 

 with its parents. Now, in the development of this cell into 

 a new animal, food is necessary. So with the fertilized egg 

 cell there is, in the case of most animals th.at lay eggs, a greater 

 cr less amount of food matter — food yolk, it is called — gath- 

 ered about the germ cell, and both germ cell and food yolk 

 are inclosed in a soft or hard wall. Thus is composed the 

 egg as we know it. The hen's egg is as large as it is because 

 of the great amount of food yolk it contains. The egg of a 

 fish as large as a hen is much smaller than the hen's egg; it 

 contains . less food yolk. Eggs (Fig. 132) ma}^ vary also in 

 their external appearance, because of the different kinds of 

 membrane or shells which may inclose and protect them. 

 Thus the frog's eggs are inclosed in a thin membrane and 



