14 THE PHENOMENA OF LIFE 



tion of external irritation, as might lead one, were this fact taken by itself, to 

 regard them as sentient beings. Inherent power of movement, then, al- 

 though especially characteristic of animal nature, is, when taken by itself, no 

 proof of it. 



Cell Differentiation and the Functions of Organized Cells. As we 

 proceed upward in the scale of life from the unicellular organisms, we find 

 another phenomenon exhibited in the life history of the higher forms, namely, 

 that of development. The one-celled ameba comes into being derived from 

 a previous ameba; it manifests the properties and performs the functions of 

 its life which have been already enumerated. In the higher organisms it is 

 different. Each, indeed, begins as a single cell, but the cells which result from 

 division and subdivision do not form so many independent organisms, but 

 adhere in one differentiated community which ultimately forms the complex 

 but co-ordinated whole, in man the human body. 



Thus, from the ovum, or germ cell which forms the starting-point of ani- 

 mal life, in a comparatively short time there is formed a complete membrane 

 of cells, polyhedral in shape from mutual pressure, called the Blastoderm; and 



FIG. 13. Transverse Section through Embryo Chick (26 hours), a, Epiblast; b, mespblast; 

 c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; e, primitive 

 groove; /, dorsal ridge. (Klein.) 



this speedily differentiates into two and then into three layers, chiefly from 

 the rapid proliferation of the cells of the first single layer. These layers, 

 figure 13, are called the Epiblast, the Mesoblast, and the Hypoblast. In the 

 further development of the animal it is found that from each of these layers is 

 produced a very definite part of the completed body. For example, from 

 the cells of the epiblast are derived, among other structures, the skin and the 

 central nervous system; from the mesoblast the muscles and connective 

 tissue of the body, and from the hypoblast the epithelium of the alimentary 

 canal, some of the chief glands, and so on. 



It is obvious that the tissues and organs so derived will exhibit in a varying 

 degree the primary properties of protoplasm. The muscles, for example, 

 derived chiefly from certain cells of the mesoblast are particularly contractile 

 and respond to stimuli readily, while the cells of the liver, although possibly 

 contractile to a certain extent, have to do chiefly with the processes of nutrition. 



