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 j 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. The mammals and 

 some other animals have this habit. When such an ani- 

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

 born. When the developing animal issues from an egg 



