CELLS AND ORGANISATIONS 115 



cell. It feeds and grows, divides and multiplies, into 2, 

 into 4, into 8, 16, 32, 64, 128, 256, or as in mammalia 

 and amphibian eggs, into 2, 4, 8, 12, 16, 24, 32, 48, 64, 

 96, 160. A globular mass of cells is constructed called 

 the mulberry germ. By and by there is an inversion of 

 the cells. At one point they begin to sink, and a cup 

 is formed. At the same time the inner ones become 

 rounder, the outer longer the latter also smaller and 

 more numerous and darker. The protoplasm of the 

 cells differs. Chemical, physical, and morphological 

 changes are produced, and thus differentiation, which 

 plays so important a part, begins. This is the process 

 in the lower animal forms. In the higher the processes 

 are somewhat different, but in all different layers of cells 

 are produced, distinguished by different characteristics. 

 The two germ layers next multiply into four, which 

 assume the shape of a tube. They are called the skin 

 sensory layer, the skin fibrous, the intestinal fibrous, the 

 intestinal glandular. According to Haeckel I. The skin 

 sensory layer produces (1) the outer skin, and in the 

 higher vertebrates the hair, nails, and the sweat and 

 sebaceous glands; (2) the central nerve system, and 

 the medullary or spinal canal. During the development 

 of the individual, it moves gradually inward, so that at 

 length it is situated internally, surrounded by muscles 

 and bones and other parts. The primitive kidney is also 

 probably from it. It eventually takes a deeper place 

 within the body. II. The second, or skin fibrous layer, 

 yields (1) the leather skin, the firm fibrous covering 

 which contains the nerves and blood-vessels of the skin ; 



(2) the great masses of the muscles of the whole trunk ; 



(3) the inner skeleton, which is specially characteristic of 

 vertebrates, the articulated vertebral column, also all the 

 bones, cartilages, and ligaments, which form the vertebral 



