TISSUE FORMATION. 57 



strata of the earth's crust. By experiments in molec- 

 ular coalescence similar forms have been produced arti- 

 ficially. 



Spicules, like those in the skin of certain marine ani- 

 mals, have also been formed by molecular coalescence, 

 as well as laminated plates like cuttle-fish bone. It is 

 quite probable that many calcareous deposits in tissues, 

 as in the shell of the bird's egg, in the scales of fishes, 

 as well as in bone and teeth, may be thus accounted for. 

 The presence and contact of living colloid matter modi- 

 fies the ordinary laws of crystallization, and produces 

 forms differing according to the endowment of the bio- 

 plasm. 



6. In vegetables most of the organs are composed of 

 cellular tissue, or a congeries of cells. The surface of the 

 cell, which originates by fission from bioplasm, is 

 changed into membrane^ or 



cell-wall, while a nucleus, (one 

 or more,) now generally re- 

 garded as a concentration of 

 vital power, appears inside. 

 Within the nucleus, another 

 spot, the nucleolus, is some- 

 times seen. (Fig. 7.) The 

 cell itself presents the ap- 



FIG. 7. Vegetable cell, with nucleus 



pearance of a bladder full of and nucieoius. 



fluid or semi-fluid material, in the midst of which the 



nucleus is visible. 



7. Many simple vegetable forms consist of a single 

 cell, the membranous wall of which is a species of formed 

 material called cellulose, a substance analogous to starch. 



