80 ANIMAL LIFE 



fusion of two germ cells, a sperm cell from a male indi- 

 vidual of the species and an egg cell from a female indi- 

 vidual of the species. The single cell thus formed is called 

 the fertilized egg cell, and its subsequent 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, and sometimes a 

 certain amount of warmth. So with the fertilized egg cell 

 there is, in the case of all animals that lay eggs, a greater 

 or 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 be- 

 cause 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. 38) may 

 vary also in their external appearance, because of the dif- 

 ferent kinds of membrane or shells which may inclose and 

 protect them. Thus the frog's eggs are inclosed in a thin 

 membrane and imbedded in a soft, jelly-like substance ; 

 the skate's egg has a tough, dark-brown leathery inclosing 

 wall ; the spiral egg of the bull-head sharks is leathery and 

 colored like the dark-olive seaweeds among which it lies ; 

 and a bird's egg has a hard shell of carbonate of lime. But 

 in each case there is the essential fertilized germ cell ; in 

 this the eggs of hen and fish and butterfly and cray-fish and 

 worm are alike, however much they may differ in size and 

 external appearance. 



49. Embryonic and post-embryonic development. Some 

 animals do not lay eggs, that is they do not deposit the fer- 

 tilized egg cell outside of the body, but allow the develop- 

 ment of the new individual to go on inside the body of the 

 mother for a longer or shorter period. All the mammals 

 and some other animals have this habit. When such an 

 animal issues from the body of the mother, it is said to be 

 born. "When the developing animal issues from an egg 



