THE GROWTH OF ANIMALS 19 



How animals begin. — Every animal/ no matter how 

 large or how small, begins as one cell, and in this all ani- 

 mals are alike. 



This one cell, in the majority of animals, is known as 

 the egg cell, or ovum. The egg cell itself is very small; 

 but in many cases, especially ia the case of those animals 

 that lay eggs, the egg cell is inclosed in an enveloping mem- 

 brane, which in turn may be inclosed ia a hard, thick shell. 

 The membrane and shell have withia them, in addition 

 to the egg cell, a considerable quantity of food, known as 

 the yolk. The egg cell, with its food yolk and membrane 

 and shell (when the latter is present), constitutes what we 

 know as an egg. Eggs vary greatly in size, owing to the 

 difference in the quantity of food yolk they contain. For 

 example, a hen's egg is much larger than the egg of a pond 

 snail, because the former contains much more food than 

 the latter. 



The majority of animals lay eggs, sometimes on land {e.g. 

 bird, turtle, etc.), sometimes in water {e.g. fish, frog, etc.), 

 but sometimes the eggs are retained in the body of the 

 parent animal. Moreover, most of the mammals retain 

 the egg cell and allow it to develop into an embryonic ani- 

 mal within the body of the mother. But whether the egg 

 cell is deposited outside of the body of the mother, or 

 whether it is retained within the body of the mother for a 

 certain period, it undergoes similar changes in its develop- 

 rrient into an adult animal. 



How cells increase in number. — We have learned in the 

 foregoing paragraph that all animals begin as one cell. Yet 

 the animals with which we are most familiar consist of 



' This may not hold strictly true m case of the hydra when it 

 reproduces by budding. 



