30 MANUAL OF PHYSIOLOGY. 



stant activity of animals, the amount laid by never attains any- 

 thing like that found in the store textures of vegetables, where 

 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 substance is 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 eye-ball the 

 cells are filled with a number of granules composed of a substance 

 very rich in carbon, 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. 



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- 



FIG. 



-me 



ch, 



Tr:iiisver>e section of Blastoderm, showing the elements in the earlier st;i.u;e. 

 of tlu- tli-Yi-li.|nnent. A,epiblast; B, nie-oblasi ; ( ', 



toplasm in the earlier periods of their development and differentia- 

 tion. All cells may then be divided into two chief types, the indif- 

 ferent and the differentiated. 



Under the category of indifferent cells may be placed all such 

 a- ivtain the characters of the first embryonic cell and have not 

 acquired any special structure or property by which they can be 



