264 ON GROWTH AND OVERGROWTH 



lying this process of growth from the earliest stages of the embryo 

 upwards, but further, applying this biological law to the subject 

 of pathology, we gain a deeper and a fuller comprehension of 

 certain matters bearing upon regeneration and degeneration of 

 tissues, and, what is more, bearing very directly upon certain of 

 the phenomena of tumour growth. 



While the individual, formed as he is of a marvellous complex 

 of various tissues, is a reproduction of the tissues and organs 

 present in his paternal and maternal ancestors, he is the outcome 

 and development, not of the combined fully developed tissues, 

 nor again of any one highly differentiated cell or cell compound, 

 but of a single undifferentiated cell — the fertilized ovum — a 

 cell neither of the component parts of which, ovum or sperma- 

 tozoon, has ever through the whole course of the ages been 

 derived from other than similar undifferentiated cells ; or, in 

 other words, these apparently simple germ cells are capable of 

 giving rise to the whole series of cells forming the whole mass 

 of tissues from the simplest connective up to the most highly 

 differentiated nerve tissue. 1 If we study the process of the 

 development of the embryo, whether in plant or animal life, we 

 see again a somewhat similar phenomenon. 



" Mother Cells " and Tissue Growth 



From a very early period in such growth we recognize that 

 certain cells alone appear to be actively dividing and to be 

 actively proliferative, whereas other cells, the products of these, 

 while they take on characteristic appearances, do not thus divide. 

 Very early indeed in the developing embryo we recognize this 

 existence of what may be termed " mother cells," — cells which 

 themselves remaining embryonic in type give rise by division 

 to other cells which assume more highly differentiated characters. 

 And this is true not only of animal organisms, but still more 



1 Throughout this article I refer to cytoplasmic and not nuclear differentia- 

 tion. It is possible that while the cell body of these germ cells is relatively 

 undifferentiated, the nucleus is peculiarly highly elaborated. We have, indeed, 

 not a few indications that this is the case. Thus it may be that nuclear and 

 cytoplasmic differentiation, in germ and " mother " cells, and in specific 

 tissue and " daughter " cells respectively, are in inverse ratio. We have not 

 as yet reached a point at which any definite statement can be made in this 

 connexion. 



