6o 



HISTOLOGY 



then becomes fixed in its permanent form. This permanent form varies 

 in different plants, its highly developed form being the wood-cell, which is 

 a cell in which the cytoplasm has developed the 

 w cell- wall out of cellulose and then has added an- 

 other and more efficient material, xylem, that, in 

 the aggregate, forms the ordinary wood of the 

 larger plants and trees (Fig. 61). 



One other example shows us a simple rigid con- 

 nective-tissue cell that is constructed to perform its 

 functions in a slightly different way. The noto- 

 chord of the vertebrates is an organ that is typi- 

 cally an organ of rigidity (Fig. 62). It is composed 

 of cells that during their development form a single 

 large vacuole in their center to secure a large sur- 

 face on a small body of cytoplasm. The outer 

 portion of this cytoplasm secretes a shell of dense, 

 firm material, whose strength, in connection with 

 that of all the other cells around it, gives to the 

 FIG. 61. A-c. Three notochord its characteristic firmness. In the pro- 

 <TromSxRAs^BTOGER cess f vacuolization the cytoplasm is pushed from 

 after SCHENCK and the center to the periphery, where it forms a shell 

 SCHIMPER.) that secretes the ce ii_ wa u. This cell-wall is the 



cell-organ of rigidity. The cytoplasm inside of this connective-tissue 

 shell is much thicker at one point 

 than anywhere else, and the nu- 

 cleus is located in this thickened 

 mass. This form is a temporary 

 embryonic one and has no re- 

 newal process. 



It should be noticed in all the 

 above examples that the vacu- 

 oles, which play such an impor- 

 tant part, are filled with a fluid 

 which, for a better knowledge of 

 its use and constitution, will be 

 called the cell-sap. 



We now turn our attention 

 to what is an extremely primi- 

 tive but comparatively rare form 

 of rigid connective tissue : This 

 is the spicule-forming cell of 

 certain sponges (Fig. 63). Many cells of the mesoglcea of such 

 a sponge form in their cytoplasm a tiny pointed rod of calcium 



