DEVELOPMENT OF ANIMALS 



617 



gelatinous masses of frogs' eggs attached to sticks or water weeds in 

 shallow ponds. Collect some and keep them in a shallow dish in a win- 

 dow at home until they hatch. Make experiments to learn whether 

 temperature affects the development of the eggs in any way. Place 

 eggs in dishes of water in a warm room, in a cold room, and in the 

 refrigerator. Make observations for several weeks as to the rate of 

 development of each lot of eggs. Also try placing a large number of 

 eggs in one dish, thus cutting down the supply of available oxygen, and 

 in another dish near by, under the same conditions of light and heat, 

 place a few eggs with plenty of water. Do both batches of eggs develop 

 with the same rapidity? In all these experiments be sure to use eggs 

 from the same egg mass, so as to make sure that all are of the same age. 



egg and sperm 



lay 



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middle cronctc£s 

 lover Skeleton 

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 v excretory - 'system 



layer [r.Sfpirator/ sy»t«w. 



Development of animals. Many-celled animals are formed 

 in much the same way as are many-celled seed plants. A com- 

 mon bath sponge, an earthworm, a fish, or a dog, — each of 

 them begins life as 

 a fertilized egg cell. 

 As in the flowering 

 plant, this cell was 

 formed by the union 

 of two other cells, 

 a tiny (usually mo- 

 tile) cell, the sperm, 

 and a larger one, 

 the egg. After the 

 egg is fertilized by 

 a sperm cell, it 

 splits into two, then 

 into four, then into 

 eight, then into six- 

 teen cells, and so 

 on. As the number 

 of cells increases, a 

 hollow ball of cells 

 called the blastula is formed ; later this ball sinks in on one side, and 

 a double- walled cup of cells, called a gastrula, results. Practically 

 all animals pass through the above stages in their development 

 from the egg, although these stages are often not plainly seen be- 

 cause of the presence of food material (yolk) in the egg. 



blastula 



All animals pass through the above stages in the process of 

 their development from an egg. 



