MORE COMPLICATED LIFE CYCLES 135 



higher in the scale of animal forms, such as the hydroids and jelly fish. 

 A very small fragment, indeed, of a hydra will reproduce the entire 

 animal, but one cell of the hydra will not do so; each of the two germ 

 layers must be represented in the small piece. In worms and in still 

 higher forms of the invertebrated animals this power to regenerate 

 the entire animal decreases pari passu with the differentiation of the 

 animal, and although not absolutely true, it may be stated in general 

 that the higher the differentiation the less is the power to regenerate 

 lost parts. In other words, something is lost from the highly differ- 

 entiated somatic cells, something which is segregated in the germ cells 

 and something which we find in each cell of the lower forms of inver- 

 tebrates, but most widespread in the unicellular protozoa. It has to 

 do with the racial characters of the organism, that is, with the germ 

 plasm. In hydra and in some of the worms the cells retain enough of 

 this germ plasm to reproduce the entire organism, but in the mammals 

 the somatic cells have so nearly lost this germinal power that regen- 

 eration of an organ or limb is no longer possible, and is limited to the 

 mere repair of an injury. In this sense, therefore, Weismann's claim 

 that natural death is the penalty higher animals must pay for differ- 

 entiation is justified. 



The so-called "noble" cells (Metchnikoff) of the body, that is, the 

 cells of brain, liver, kidney, and other important centres of physio- 

 logical activity, are somatic cells in which this regenerative power is 

 reduced to a minimum; the potential of germinal activity in them is 

 less than in connective-tissue cells, and after an injury their power of 

 repair is less than that in connective-tissue cells. This is seen in the 

 fact that a wounded epithelium is repaired less by the proliferation of 

 the neighboring epithelial cells than by the adjacent connective tissue, 

 and the "scar" tissue which results is composed of these "baser" 

 cells. 



Like the physiological activities of paramecium, all somatic cells of 

 the body are endowed with a certain potential of physiological activity, 

 and like paramecium, when exhausted the particular function of those 

 cells ceases; they have reached the limit of their activity, and when 

 enough of them are so worn out a general impairment of the body 

 functions results. This condition of the exhausted cells may be 

 relieved by stimulants which, we imagine, may come from the general 

 body itself, or from artificial treatment, as in the case of paramecium. 

 But wc have no reason to believe that in the human somatic cells this 

 stimulation can be repeated indefinitely. If in the generalized proto- 

 zoon there comes a time in which the potential of germinal activity 

 of the cell gives out, how much more probable would it be that the 

 somatic cells, with their low potential of germinal activity, likewise 

 fail to respond to the stimulants. Unable to reproduce by division, 

 with their potential of physiological activity reduced to a minimum, 



