REPRODUCTION AND DEVELOPMENT OF ANIMALS 299 



nourishment for each developing animal. This nourishment, 

 when attached to the egg cell and enclosed in an egg covering, 

 is known as yolk. 



349. The Care of the Young. As a rule, the animals 

 which have small egg? produce many. Worms, lobsters, 

 insects, fishes, and frogs deposit 

 small eggs in immense numbers; 

 birds and reptiles produce larger 

 eggs in fewer numbers. The care 

 given to the eggs is greater when 

 a small number is produced. Some 

 reptiles and all birds have eggs 

 with a tough or hard covering. 

 Fishes, worms, and other water 

 animals have soft, jelly-like eggs. 



The development of the embryo 

 begins in the same way as in the 

 plant: the egg cell divides and 

 subdivision .continues until the 

 young animal is completely 

 formed. In the case of the 

 highest class of vertebrate 

 animals, the egg cell remains and 

 is nourished in the body of the female for a longer or 

 shorter time. After the birth, the female continues to 

 furnish protection and nourishment' for weeks and even 

 months. This nourishment, the milk, contains proteid, fat, 

 sugar, and mineral matter dissolved in water. This is all 

 the food the young animal needs until it is able to digest 

 solid foods. 



Birds care for their eggs during the period of development 

 of the embryo, and a few species of fishes make nests and 

 guard them, but usually the eggs of fishes are left to develop 

 alone. The same is true of reptiles and insects. As soon as 

 the food provided in the yolk is used up, the eggs hatch and 



FIG. 157. REPRODUCTIVE 

 CELLS (magnified) 



a is an egg cell of a frog, 

 b, a sperm cell. The union of 

 such cells results in the forma- 

 tion of an egg, such as is shown 

 in B, Fig. 158. 



