558 THE HUMAN BODY 



ually built up into Territories and States. The new individual 

 in the political world began with little differentiation ; it consisted 

 of units, separated from older and highly organized societies, and 

 these units at first did pretty much everything, each man for him- 

 self, with more or less efficiency. As growth took place develop- 

 ment also occurred; persons assumed different duties and per- 

 formed different work until, finally, a fully organized State was 

 formed. Similarly, the body of one of the higher animals is, at 

 an early stage of life, merely a collection of undifferentiated cells, 

 each capable of multiplication by division, and more or less re- 

 taining all its original protoplasmic properties; and with no spe- 

 cific individual endowment or function. The mass (Chap. Ill) 

 then slowly differentiates into the various tissues, each with a 

 predominant character and duty; at the same time the majority 

 of the cells lose their primitive powers of reproduction, though 

 exactly how completely is a problem not yet sufficiently studied. 

 In adult Vertebrates it seems certain that the white blood cor- 

 puscles multiply by division: and in some cases (in the newts or 

 tritons, for example) a limb is reproduced after amputation. 

 But exactly what cells take part in such restorative processes is 

 uncertain; we do not know whether or not the old bone corpuscles 

 left form new bones, old muscle-fibers new muscles, and so on. 

 In Mammals no such restoration occurs; an amputated leg may 

 heal at the stump but does not form again. In the healing proc- 

 esses the connective tissues play the main part, as we might 

 expect; their cellular elements being but little modified from 

 their primitive state can still multiply and develop. New blood- 

 capillaries, however, sprout out from the sides of old, and new 

 epidermis seems only to be formed by the multiplication of 

 epidermic cells; hence the practice, frequently adopted by sur- 

 geons, of transplanting little bits of skin to points on the surface 

 of an extensive burn or ulcer. In blood-capillaries and epidermis 

 the departure from the primary undifferentiated cell is but slight ; 

 and, as regards the cuticle, one of the permanent physiological 

 characters of the cells of the rete mucosum is their multiplication 

 throughout the whole of life; that is a main physiological char- 

 acteristic of the tissue : the same is very probably true of the pro- 

 toplasmic cells forming the walls of the capillaries. When a highly 

 differentiated tissue is replaced in the body of mammals after 



