54 THE EVOLUTION OF MAN. 



fclie skin, or dermal layer, and the flesh, or muscular layei. 

 From the uppermost of these two lamellse, the skin-layer, 

 are formed the outer skin, the covering of the body, and the 

 central nervous system, the spinal cord, the brain, and 

 the organs of sensation. From the lower, or flesh-layer, 

 the muscles, or fleshy parts, the internal or bony skeleton, — 

 in short, the organs of motion, arise. Secondly, the lower, 

 or vegetative germ-layer, parts in the same way into two 

 lamellse, which Baer distinguishes as the vascular and the 

 mucous layer. From the outer of the two, the vascular 

 layer, proceed the heart and the blood-vessels, the spleen, 

 and the other so-called blood-vessel glands, the kidneys, 

 and the sexual glands. Finally, from the lowest, and fourth 

 or mucous layer, arises the inner alimentary membrane of 

 the intestinal canal, with all its appendages, liver, lungs, 

 salivary glands. Baer traced the transformation of these 

 four secondary germ-layers into tube-shaped fundamental 

 organs as ingeniously as he had successfully determined 

 their import and their formation in pairs by the segmen- 

 tation of the two primary germ-layers. He was the first 

 to solve the difiicult problem as to the process by which 

 the entirely different body of the vertebrate develops from 

 this flat, leaf-shaped, four-layered original germ ; the process 

 was the transformation of the layers into tubes. 



In accordance with certain laws of growth, the flat 

 layers bend, and become arched ; the edges grow towards 

 each other so that the distance between them is continually 

 decreased ; finally they unite at the point of contact. By 

 this process the flat intestinal layer changes into a hollow 

 intestinal tube ; the flat spinal layer becomes a hollow 

 spinal tube, the skin-layer becomes a skin-tube, etc. 



