336 ON GROWTH AND OVERGROWTH 



simplest explanation. Where are we to place these tumours ? 

 The cells that give origin to them have not the power, even under 

 the most favourable circumstances, to reproduce all the different 

 forms of tissue ; they have not the potentiality to form, even in 

 the most favourable circumstances, the complete individual. 

 And, on the other hand, there are no signs in these tumours of 

 a combination of tissues derived from all three germinal layers. 

 There may, indeed, be only the representative of one layer, the 

 mesoblast. But we have in this order of tumour two or more 

 distinct forms of tissue, in addition to the connective tissue 

 stroma. They are not teratomas, they are not typical blastomas. 



It is here that the terminology introduced by the embryologist 

 Barfurth, in his Handbuch der Embryologie, appears to me to 

 be of singularly high value and to afford the basis of an adequate 

 classification. Barfurth has pointed out that, following the 

 development of the typical ovum, one recognizes a first stage in 

 which each individual blastomere or product of the cell division 

 of that ovum may be shaken apart from its fellows, as has been 

 done by Driesch, Roux of Breslau, E. B. Wilson, Morgan, Jacques 

 Loeb, and many others, and each cell so separated has the power 

 of giving origin to a complete, even if somewhat dwarfed, in- 

 dividual. Such cells he terms totipotential. 



Following upon this stage, he notices that as the cells of the 

 dividing ovum develop into the different germinal layers these 

 cells no longer possess the individual power of giving origin to 

 the complete individual. But, obviously, in this next stage the 

 individual cells of the primitive streak region in differentiating 

 into epiblast, mesoblast, or hypoblast, according to their position 

 become the ancestors not of a single tissue, but of various groups 

 of tissues. If epiblastic in position, they give origin to the 

 cutaneous, glandular, and nervous elements ; even if at an early 

 stage they give off cells which become mesoblastic, and so origin- 

 ate members of the group of what we term familiarly the con- 

 nective elements. Nor is this capacity to give origin to more 

 than one form of tissue confined only to the familiar three-layer 

 period. The cells may be regarded as retaining this capacity 

 throughout the whole of what Ballantyne has termed the germinal 

 period of the embryo, until, that is, they become so far differen- 

 tiated by division and relative site that they give origin to the 

 matrices and anlagen of the different organs of the body. We 



