356 



GENERAL BIOLOGY 



Normal regenera- 

 tion for the main- 

 tenance of the body. 



— Regeneration o f 

 lost parts is but a 

 manifestation of the 

 power of growth ap- 

 plied in abnormal 

 circumstances. It is 

 a very noticeable 

 thing when, by some 

 mishap with tools or 

 machinery, we knock 

 off a finger nail and 

 have to grow a new 

 one; but physiologi- 

 cally it is not very 

 different from getting 

 our hair cut and hav- 

 ing it grow out again. 

 Our epidermis is con- 

 tinually being shed 

 from the surface and 

 new cells are con- 

 tinually growing up 

 from below. An ex- 

 cellent example of the 

 renewal of tissues 

 inside the body is 



Fig. 206. Growth of digestive epithelium in a dragonfly nymph (Gomphus). 



a, the aUmentary canal as a whole; g, gill chamber, /and 2, Main divisions of 

 the intestine, x, nephridia (Malpighian tubules) :v, stomach, s, crop. 



b, Section of a bit of the stomach wall: k, digestive epithelium. /, longi- 

 tudinal muscle fibres; m, longitudinal muscle layer; n. (as in all the 

 following) a nest of cells for replacement of the functional epithelial cells, 



c, the same as 6, after fasting two weeks: note the accumulation of di- 

 gestive secretion as shown in height of functional cells. 



d, a dissociation preparation of part of one of the replacement cell nests. 



e, the discharge of the digestive secretion after feeding, o, and p, globules 

 of discharge: the oldest of the functional cells are thus thrown off bodily. 



f, the replacement of the discharged cells with new ones from the cell nests 



n, n, n. Note the new (clear) cells crowding to the surface. 



