AND PHYSIOLOGY. 



21 



65. It is in these 

 ways that most of the 

 tissues of the body are 

 built up, and the animal 

 enlarges to its full size. 

 It has been maintained 

 that all the tissues orig- 

 inate in cell develop- 

 ment. But, in some 

 cases, simple fibers and 

 membranes seem to be 

 formed directly out of 

 an organizable s u b - 

 stance, without the in- 

 tervention of cells. 



66. Vital Force of 

 Cells; Chemical Trans- 

 formations; Vitalization of Hie Cells; Change of Form; 

 Development of Nerve Force.— The multiplication of cells 

 is one of the manifestations of the vital force inherent in tham. 

 Another is the chemical transformations exerted upon the con- 

 tents of the cell in some instances, whereby new products 

 are generated. Another is the vitalization of a portion of 

 the cell contents, whereby they are able to produce new 



. cells. Another is permanent changes of form in connection 

 with growth. Another is temporary changes of form, ac- 

 companied with sensible motion, as in the oscillatory move- 

 ments of the leaves of Hedysarum gyrans. and the fold- 

 ing of the leaves of the Mimosas upon touch. Finally, the 

 development of nerve force from cells, by which all the bodily 

 operations may be modified, and which is intimately connected 

 with mental agency. 



67. Periods in the Life of the Cells. — In cell life there is a 



Fig. 12. 



65. Show how the tissues of the body are made up. 66. Mention the different changes 

 which cells undergo. 



