284 BIOLOGY 



final result is that the embryo has an ectoderm of smaller cells 

 on the outer side, an endoderm of larger cells on the inner side, 

 and a mesoderm between the outer and the inner layer. These 

 three layers of cells remain distinct, and are destined for dif- 

 ferent purposes in the subsequent life of the animal, each one 

 of them developing into certain organs only. The organs that 

 are developed from the three layers are as follows : 



The mesoderm. From the mesoderm develop the muscles, 

 the bones, the heart, and the blood vessels, the lining of the body 

 cavity, the outer layer of the alimentary canal, the mesentery 

 which holds the alimentary canal in position, and the repro- 

 ductive system. 



The endoderm. From the endoderm develop the alimentary 

 canal, the glands around the mouth, the lungs, the pancreas, 

 and the liver. The muscles which form the wall of the alimen- 

 tary canal are developed from the mesoderm, but the lining of 

 the digestive tract, with all its glands, which secrete the digestive 

 juices, is formed from the endoderm. 



The ectoderm. The ectoderm gives rise to the skin, includ- 

 ing the epidermis and the dermis. It also grows inward to 

 line the mouth and the extreme posterior end of the alimentary 

 canal. The ectoderm also gives rise to the nervous system, 

 with all of its parts, including the brain, the spinal cord, the 

 nerves, and all of the sensory organs, like the eyes, the ears, 

 organs of smell and touch. 



It will be seen that the alimentary canal is made of three 

 parts : the anterior end is formed by the infolding (invagination) 

 of the ectoderm, the infolded part forming the mouth or buccal 

 cavity; the posterior end is also formed by an invagination of 

 the ectoderm, which forms the cloacal chamber; the rest of the 

 canal is formed from the endoderm. These three parts are 

 called the foregut (stomodceum) , the midgut (mesenteron) , and 

 the hindgut (proctodceum) . Similar relations are found in other 

 vertebrates and also in the lower animals as well. 



Layers similar to those described are found in the embryos 



