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GENERAL ZOOLOGY 



ectoderm, and a layer lining the inturned region, called the 

 endoderm. This stage, when the embryo consists of two 

 layers of cells, is called a gastrula. 



Later Development. Between the ectoderm and endoderm 

 a third layer, or group, of cells forms (Fig. 73, G). These 

 are called the mesoderm. From these three layers of cells 

 the fully formed animal grows. The cells which differed 



Fig. 73. Cleavage of the egg and early development of the starfish. 



(Enlarged diagrams) 



A, egg before development starts ; B, two cells resulting from the division of 

 the fertilized egg ; C, four-cell stage ; D, eight-cell stage ; E, section through 

 the blastula stage ; F, section through gastrula stage ; G, a later gastrula, show- 

 ing how mesoderm is formed 



little from one another except in size become specialized 

 for various work. This specialization began with the forma- 

 tion of the gastrula, for the cells of the endoderm form a 

 sac, which in some animals serves as a stomach, or later 

 develops into a digestive system. From the outer layer, or 

 ectoderm, the skin and the nervous system develop. The 

 lining of the alimentary canal comes from the endoderm. 

 The most of the rest of the body is formed from mesoderm. 

 This includes the muscles, excretory system, circulatory 

 system, and reproductive organs. 



