METAZOA. 319 



the ectoderm and entoderm of the Coelenterata. The study 

 of the development of these three-layered animals shows a 

 very interesting fact. At first there are but two layers in 

 the body, and later the mesoderm develops between these 

 two. In other words, all of the higher Metazoa pass through 

 a stage in which they exhibit a coelenterate condition. 



These three layers reach their highest condition in the 

 Vertebrates, and it may be interesting to see how all the 

 various structures which have been studied in a shark or in 

 a rat are related to these layers. 



To the ECTODERM belong the outer layer of the skin, 

 the outer layer of scales, the hair, feathers, sweat-glands, 

 the enamel of the teeth, the nervous system, the sensory 

 portions of sensory organs, and the lens of the eye. 



The ENTODERM furnishes the lining of the alimentary 

 canal, the notochord, gills, tracheal lining, lungs, liver, 

 pancreas, urinary bladder. 



The contributions of the MESODERM to the body are more 

 extensive. They include the deeper layers of the skin, 

 fat, muscles, connective-tissue, cartilage, bones, ligaments, 

 blood-vessels, blood, the lining (pleural and peritoneal mem- 

 branes) of the body-cavity, the deeper layer of the scales, 

 the dentine of the teeth, the outer layers of the alimentary 

 canal, and the reproductive and excretory organs and their 

 ducts. 



If we study any part of any one of the animals already 

 dissected or mentioned under the higher powers of the 

 microscope having first treated it so as to bring out details 

 we will discover another fact of great importance. Every 

 one of these animals will be found to be made up of small 

 parts, essentially like each other, just as the wall of a build- 

 ing is built up of separate bricks. These separate parts or 

 elements of the Metazoa are known as cells. Each one of 



