MIDDLE GERM-LAYERS. 235 



and limbs, as well as the inner skeleton, or bony frame- 

 work of the body, arise from the second germ-layer, or the 

 outer middle layer, which is called the skin-muscular 

 layer, or the skin-fibrous layer. The muscles and vascular 

 membranes, which first surround the inner cellular canal 

 of the intestine and its glands, and which accomplish the 

 digestive movements of the throat (pharynx), oesophagus, 

 the stomach, and the various other parts of the intestinal 

 canal, are all produced from the third germ-layer, the 

 inner middle layer, which is called the intestinal-muscular 

 layer, or the intestinal-fibrous layer; the heart and the 

 most important blood-vessels also originate in this. The 

 two middle layers, therefore, especially provide those cell- 

 strata which are employed in the formation of the fibrous 

 coverings, and of the flesh or muscles. The cells of the 

 second layer change into the flesh and the bony framework 

 of the trunk; the cells of the third layer change into the 

 muscles and the fibrous coverings of the intestinal canal. 

 Both middle or fibrous layers are therefore called muscular, 

 or flesh-layers ; the outer is called the skin-muscular layer, 

 because it lies on the first secondary layer, the skin-sensory 

 layer ; the inner is called the intestinal-muscular layer, as 

 it lies next to the fourth secondary layer, the intestinal- 

 glandular layer (Fig. 50). 



Baer was the first naturalist who recognized and clearly 

 distinguished the four secondary germ-layers of the higher 

 animals. He did not, however, fully understand their 

 origin and their wider significance, nor was he quite 

 right in his explanation of the details of their respective 

 purposes. But in the main, their significance did not 



escape him, and he even expressed that view of the origin 



18 



