CHAPTER II. 



THE FUNCTIONS OF ORGANIZED CELLS. 



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

 we find that another phenomenon is exhibited in the life history of the 

 higher forms, namely, that of Development. An amoeba comes into be- 

 ing derived from a previous amoeba; it manifests the properties and 

 performs the functions of its life which have been already enumerated; 

 it grows, it reproduces itself, whereby several amoebae result in place of 

 one, and it dies. It cannot be said to develop, however, unless the for- 

 mation of a nucleus can be considered as an indication of such a process. 

 In the higher organisms it is different; they, indeed, begin as a single 

 cell, but this cell on division and subdivision does not form so many 



Fig. 18. Transverse section through embryo chick (26 hours), a, Epiblast; 6, niesoblast; c, 

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

 /, dorsal ridge. (Klein.) 



independent organisms, but produces the material from which, by devel- 

 opment, the complete and perfect whole is to be derived. Thus, from 

 the spherical ovum, or germ, which forms the starting-point of animal 

 life and which consists of a protoplasmic cell with a nucleus and nucle- 

 olus, in a comparatively short time, by the process of segmentation which 

 has been already mentioned, a complete membrane of cells, polyhedral 

 in shape from mutual pressure, called the Blastoderm, is formed, and 

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

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

 ure called the Epiblast, the Mesoblast, and the Hypoblast (fig. 18). 

 It is found in the further development of the animal that from each 

 of these layers is produced a very definite part of its completed body. 



For example, from the cells of the epiblast are derived, among other 



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