268 ON GROWTH AND OVERGROWTH 



known as sarcoblasts, relatively large cells with abundant proto- 

 plasm, the nuclei of which tend to proliferate ; and along one 

 aspect of, or generally around, the periphery there is gradually 

 developed first a longitudinal fibrillation, and then a transverse 

 striation. In young larval forms of amphibia, for example, 

 after injury such sarcoblasts may be seen to be present, to undergo 

 multiplication, and to give rise to typical muscle fibres ; here, 

 therefore, very clearly there are mother cells. The existence 

 of such in the adult is still a matter of dispute, for in the older 

 individuals two processes have more especially been recognized : 

 one, that in which the fully formed muscle fibres become the 

 seat of an active nuclear proliferation, and then in certain areas 

 of the fibres the striation, more especially toward the end, or 

 laterally, becomes indistinct and little buds containing nuclei 

 are given off, which become the precursors of the new growth. 

 Or secondly, as pointed out first by Weismann, there may be 

 recognized, even in perfectly normal muscle of adult animals, 

 certain occasional spindle-like bodies, consisting of short striated 

 muscle fibres, characterized by very abundant nuclei, and, 

 according to Kolliker and others, these bodies represent centres 

 from which by longitudinal division new fibres are given off. 

 Even in this case it will be seen that these bodies represent a 

 lower imperfect stage of differentiation, and the richly nucleated 

 mass may strictly be regarded as a " mother cell." 1 



Thus from a study of the process of regeneration as seen in 

 the different tissues in man and vertebrates generally, it is 

 possible to lay down the following laws : — 



1. The fully differentiated cells of a tissue proper never arise 



from cells that are themselves fully differentiated. 



2. Under the normal conditions of growth and during physio- 



logical regeneration, the fully differentiated cells would 

 seem in nearly every case to be developed from " mother 

 cells " — undifferentiated or but partially differentiated 

 cells which themselves throughout the term of life of the 

 individual never attain to the full differentiation peculiar 

 to the tissue to which they belong and which indeed 

 they produce. For these mother cells by division give 



1 [Since 1900, when this paper was written, the general consensus of opinion 

 has veered round to a belief that these muscle spindles are not associated with 

 regeneration.] 



