118 TEXTBOOK OF ZOOLOGY 



portion to orgajiize as a distinct layer between the other two. The 

 position of the mesoderm is external to the endoderm and internal 

 to the ectoderm. It nearly encircles the endoderm. Sooner or later a 

 space forms within the mesoderm, causing the outer limb of it to 

 join the ectoderm and the inner to join the endoderm. This cavity 

 is the coelom or future body cavity. From each of the germ layers, 

 particular parts of the body are derived. 



The fate of the germ layers is determined as cell division and 

 development continue. The division proceeds at different rates in 

 different regions and at different times resulting in various infold- 

 ings, outpushings, and extensions which finally bring about the 

 formation of all parts of the mature individual. The ectoderm gives 

 rise to the external surface cells or epidermis of the skin and to 

 the nervous system; the mesoderm furnishes the muscles, skeleton, 

 circulatory system, blood, excretory, and reproductive systems be- 

 sides nearly all connective tissue ; and the endoderm produces the 

 internal linings of the digestive tract, respiratory tract, and such 

 outgrowths as the liver and pancreas. 



