THE LIFE CYCLE 357 



furnished by the digestive epithelium of the dragonfly shown 

 in figure 206. New cells are constantly being formed in 

 little replacement centres at the base of the epithelial layer, 

 and the old ones, charged with the digestive secretions, are 

 thrown off at every meal, to be mixed with the food and by 

 their action upon it to dissolve it. 



The tissues of the body differ much in their capacity for 

 cell replacement; some cells like those of the lowermost 

 layer of the epidermis, retain this capacity through life, 

 others like nerve cells do all their dividing in embryonic life 

 (hence, the great size to which the brain of the higher verte- 

 brates so early attains), and have no capacity for making 

 good cell losses. But if they have lost the power of produc- 



'/Do ©.■©/ U 



Fig. 207. Diagram of cell regeneration (after Morgan). 

 a, an egg of a sea urchin that was divided as shown by 

 the oblique line; b to f its subsequent development; 

 g, the enucleate part of the egg; h, its fertilization by a 

 sperm cell; i, j, k, its subsequent development. 



ing new cells, they retain the power of repairing the old 

 ones. If a nerve fibre be severed, a new fibre may grow out 

 from the cell body at the stump of the old one. It is thus 

 that a limb regains sensitiveness after being paralyzed by 

 the cutting of a nerve. 



Regeneration in cells and in embryos. — If an egg cell be 

 divided the portion containing the nucleus may reshape 

 itself, and go on developing quite normally, as illustrated in 

 figure 207 b to /. And if the other part of the cytoplasm 

 be supplied with a nucleus, as by the addition of a sperm 

 cell of the same species (fig. 207 h) it also may develop in 

 the ordinary way. The two cells resulting from the first 

 division of the egg of a sea urchin may develop as indicated 



