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HANDBOOK OF PHYSIOLOGY. 



interior of the cellulose wall; and (2) a reticulate mass containing the 

 nucleus and occupying the cell-cavity; its interstices are filled with fluid. 

 In young vegetable cells such a distinction does not exist; a finely gran- 

 ular protoplasm occupies the whole cell-cavity (Fig. 8, A). 



FIG. 8. (A) Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm 

 inclosing a large oval nucleus, with one or more nucleoli. (B) Older cells from same plant, show- 

 ing distincc cellulose- wall and vacuolation of protoplasm. 



Another striking difference is the frequent presence of a large quantity 

 of intercellular substance in animal tissues, while in vegetables it is com- 

 paratively rare, the requisite consistency being given to their tissues by 

 the tough cellulose walls, often thickened by deposits of lignin. As an 

 example of the manner in which this end is attained in animal tissues, 

 may be mentioned the deposition of lime-salts in a matrix of intercellu- 

 lar substance in ossification. 



Morphological Development and Division of Functions. 



As we proceed upwards in the scale of life from monocellular organ- 

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

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

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

 performs functions of life which have been already enumerated; it grows, 

 it reproduces itself, whereby several amoebae result in place of one, and 

 it dies, but it can scarcely be said to develop unless the formation of a 

 nucleus can be so considered. In the higher organisms, however, it is 

 different; they, indeed, begin as a single cell, but this cell on its division 

 and subdivision does not form so many different organisms, but possesses 

 the material from which, by development, the completed and perfected 

 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 proto- 

 plasmic cell with a nucleus and nucleolus (see Fig. 4), in a comparatively 

 short time, by the process of segmentation which has been already men- 

 tioned, 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 are called the epiblast, the 

 mesoblast, and the hypoblast. 



