DEVELOPMENT OF A SIMPLE ANIMAL 



227 



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

 them begins hfe as a fertihzed egg cell. As in the flowering plant, 

 this cell was formed by the union of two other cells, a tiny (usu- 

 ally motile) 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 sixteen cells, and so on. As the number 

 of cells increases, a hollow 



ball of cells called the 

 hlas'tula is formed; later 

 this ball sinks in on one 

 side, and a double-walled 

 cup of cells, called a gas'- 

 trula, results. Practically 

 all animals pass through 

 the above stages in their 

 development from the egg, 

 although these stages are 

 often not plain to see 

 because of the presence of 

 food material (yolk) in the 

 egg. 



In an older stage the 

 body consists of three 

 layers of cells. Those of 

 the outside, developed 

 from the outer layer of 

 the gastrula, are called the 

 ec'toderm; this later gives 

 rise to the skin, nervous 

 system, etc. An inner 



Fertilized 

 egg of d 

 sea urchin 



B-celled 

 stage 



A hollow 

 ballot 

 cells. a 

 blaslula 



Eighf cells 



Thirty-two 

 cells 



The blast uld 

 begins to 

 dent and 

 form the 

 gastrula 



Cross 

 section 

 of a 

 gastrula 



Ectoderm 

 Endoderm 

 Mesoderm 

 grows here 



Development of a many-celled animal. This 

 . , „ process is called cleavage or segmentation. 



layer, developed from the 



inner layer of the gastrula, is called the en'doderm; this forms the 

 lining of the digestive organs, etc. A middle layer, called the 

 mes'oderm, lying between the ectoderm and the endoderm, gives 

 rise in higher animals, to muscles, the skeleton, and parts of other 

 internal structures. 



Asexual Growth. — There are some exceptions to the general 

 rule of development of animals stated in the last section. Some 



