80 ANIMAL LIFE 
fusion of two germ cells, a sperm cell from a ntale 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 great 
or less amount of food matter—tfood 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. 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 
