38 



MANUAL OF PHYSIOLOGY. 



the result of an entire summer's active work is put by as a pro- 

 vision for the next winter, and the fresh burst of energy which 

 follows it in the spring. 



But while the above are all more or less temporary contents of 

 cells, we have an example of a permanent deposit in them, viz., 

 Pigment. This substancais formed by the protoplasm in various 

 parts and has a special physiological use. Thus in the cells of 

 the tissue behind the retina or nerve layer of the eyeball the 

 cells are filled with a number of granules of a pigmented sub- 

 stance, which when in a sufficiently thick layer is able completely 

 to absorb any light that may fall upon it, thus preventing the 

 reflections that would otherwise occur, and which would interfere 

 with the clearness of sight. 



FIG. 5. 



Transverse section of blastoderm, showing the elements in the earlier stage of the 

 development. A,epiblast; B, mesoblast ; C, hypoblast. 



It also occurs in the skin of the negro and other races, and in 

 that of the frog and other animals, but in these its function is 

 not fully known. 



Varieties Of Cells. Great varieties of cells are found in 

 the various mature tissues of the higher animals, all of which have 

 passed through the stage of being a simple nucleated mass of pro- 

 toplasm in the earlier periods of their development and differen- 

 tiation. All cells may then be divided into two chief types, the 

 indifferent and the differentiated. 



Under the category of indifferent cells may be placed all such 

 as retain the characters of the first embryonic cell and have not 

 acquired any special structure or property by which they can be 

 distinguished from the simplest form. Such cells are the only 

 ones in the early stages of the embryo. In the adult tissues they 



