GASTRULA AND BODY LAYERS 



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and nervous system ; the endoderm forms the lining of the 

 digestive tract and the larger part of the digestive glands, 

 such as the liver and pancreas; while the mesoderm 

 develops into all of the other organs of the body, such as 

 muscles, blood, body cavity, and germ-cells. 



Fig. 72. — Early Development of a Starfish. A, B, C, D, early 

 segmentation states, E, early blastula, F, late blastula in section, 

 G, early gastrula, H, late gastrula showing budding of body cavities 

 from tip of gut (front view), /, side view of H, showing development 

 of mouth. 



Later Development. — The hydra and the related sea 

 anemones and jelly-fishes are little more than modified 

 gastrulae, and accordingly do not develop much beyond 

 this point. In the higher groups, on the contrary, the 



