HYPERTROPHY, HYPERPLASIA, METAPLASIA, REGENERATION. 99 



ences in the shape and character of the cells and the arrangement of the 

 tissue elements, these often approaching the embryonic type in form as 

 well as in character of development. Furthermore, the atypical arrange- 

 ment often seen in new-formed cells, both in regard to each other and in 

 their association with older tissues, may aid in the identification of the 

 formative process. 



Individual cells, even after having undergone marked structural 

 changes as, for example, in albuminous degeneration or after a certain 

 degree of physical injury, may be restored to a normal condition. 



After destructive injury or loss a full and complete replacement of 

 cells and tissues an occur only as the result of a proliferation of cells of 

 the same type as those to be restored. Thus a regeneration of epithe- 

 lium occurs by proliferation and growth of epithelial cells alone; regen- 

 eration of muscle by muscle cells, etc. In fact, however, in the higher 

 types of tissue, after considerable injuries with loss of substance or after 

 destructive pathological processes, complete regeneration is not common. 

 This, as we have seen, is because the highly specialized cells of the body 

 are limited in their capacity for reproduction closely to the domain of 

 physiological regeneration. What we ordinarily call healing in exten- 

 sive wounds of the more highly specialized tissues is usually a provi- 

 sional makeshift repair by means of new-formed connective tissue. 



We have seen that the regenerative capacity in the cells of the human 

 body is most marked in the less highly differentiated types of cells, and 

 that it is above all connective tissues, blood-vessels, and epithelium 

 which most frequently and most completely undergo regeneration. These 

 are relatively lowly organized tissues and serve for the maintenance or 

 protection of more highly specialized tissues, and with them regenera- 

 tion may be complete with full restoration of function. But although 

 the more highly organized tissues in man do not undergo after injury any 

 considerable regeneration, they are, when uninjured, capable, under the 

 stimulus of increased functional exercise, of compensatory hypertrophy, 

 so that the loss to the organism of similar tissue is made good, thus by 

 the structural hypertrophy and increased performance of one kidney 

 after the removal of the other. 



The capacity to regenerate lost or injured parts exists to a certain ex- 

 tent in all animals but is most marked among the lower forms. Thus if 

 an amosba be cut in two so as to leave one part with the intact nucleus, 

 this part lives and the one-celled organism is completely restored. The 

 fresh-water hydra, composed of many cells, may reproduce a large por- 

 tion of the organism from a small severed fragment. The common earth 

 worm can reproduce a severed head or tail. Crabs reproduce a whole leg 

 if the severance takes place at a particular joint. Salamanders, snails, 

 etc., can reproduce leg and tail. It is further noteworthy in this connec- 

 tion that the larvae of many lower forms, such as reptiles and insects, 

 have a much greater capacity for the reproduction of lost parts than the 

 same species have when in the adult condition. 1 



'Consult Morgan, '" Regeneration, " Columbia University, Biol. Ser.. 1901. 



