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MOTHER CELLS " 265 



obviously of plants : we have but, with Sachs, to study the 

 growing points of plants to realize the existence of these " mother 

 cells." 



Now when we pass to the fully formed individual, we must 

 still recognize this fact, which we are apt to pass over, namely, 

 that the specific cells, if I may so term them, of the different 

 tissues — that is, the highly differentiated and characteristic cells 

 of those tissues — do not themselves give rise directly, and by 

 division, to other specific cells, but that in each tissue there are 

 (1) these more or less undifferentiated mother cells, which more 

 especially have the power of proliferating, and it is the daughter 

 cells which assume the full specific properties in the different 

 tissues, or (2) if, as is seen to be the case in some instances, 

 the active cells of certain not very highly differentiated tissues 

 themselves proliferate, they only do this after a preliminary 

 reversion to a " mother cell," or more embryonic type. 



Let me here give a few examples. Immediately I mention 

 them, every one will realize that these are facts thoroughly 

 familiar. One of the simplest cases is that of the skin : in this 

 it is the lowest layer of the epidermis, the Malpighian layer, 

 whose cells undergo active proliferation, and this constantly, 

 throughout the whole period of life, and the cells which are most 

 active in the process are those of the very lowest layer — cells 

 which are of a permanently embryonic type. 



After injury, it is true, we may come across occasional mitotic 

 increase, more especially in the lower animals, in the region of 

 the prickle cells, but, when we do, we see that it is accompanied 

 by reversion of the cell to a simpler type : the prickles or 

 bridges disappear, the cell loses its connexion with the 

 neighbouring cells, and passes back to a simpler stage prior to 

 multiplication. 



In bone the case is the same. Production of bone tissue is 

 not brought about by multiplication of the pre-existing and 

 characteristic bone cells, but by osteoblastic tissue ; or, in other 

 words, by the mother cells of bone lying in the immediate 

 neighbourhood of the vessels of the inner layer of the periosteum, 

 or which course through the Haversian canals and between the 

 laminae. When through injury or otherwise a stimulation is 

 afforded, it is these mother cells which, applying themselves to 

 the pre-existing bone, multiply, and there governing the spaces 



