312 ON GROWTH AND OVERGROWTH 



nearly expresses the conception which I wish here to impress 

 on the reader, namely, that in this very earliest stage the mother 

 cells from which certain tissues are derived already have a definite 

 order and position, as constituting membranes, whereas the 

 mother cells of the other group of tissues lack this definite order 

 and exhibit no marked differentiation. 



If, now, we follow up these two orders of tissues to their full 

 development, we find that from the lining membranes are de- 

 veloped tissues of one character; from the pulp, tissues of a 

 different character. These lining membranes, from whatever 

 layer they originate, may either remain as functional membranes 

 covering an extensive surface, or they may become modified 

 so as to form the various highly specialized and specific con- 

 stituents of various organs. To reach this latter stage the 

 epithelium either undergoes extensive infolding, or sends down- 

 ward tubular processes, which branch to a greater or less extent, 

 or, again, which sends downward solid processes, as occur, for 

 example, in the development of the suprarenal. In quite a large 

 number of instances it would seem that the first process in the 

 production of the tubular glands is one of budding, or projection 

 downwards into'the underlying mesenchyme, of solid processes, 

 which subsequently became hollowed out and tubular (e.g. sudori- 

 ferous glands and renal tubules). Thus it comes to pass that 

 the various glandular organs are formed of a parenchyma originat- 

 ing from one of the primitive lining membranes and a stroma 

 of connective tissue which is of mesenchymatous origin. And 

 even in cases where there is the widest divergence from the original 

 type of lining membrane, we find that this distinction still holds, 

 that the parenchymatous cells form layers or groups into which 

 the vessels do not penetrate, and in which there is an absence of 

 stroma between the members of the cell groups, the cells being at 

 most united by bridges and by a fine cement material. While, 

 contrariwise, regarding tissues originating from the embryonic 

 pulp, we notice that in them the prominent characteristic is 

 that there is an intercellular ground substance, either homogeneous 

 or fibrillated, separating the specific cells of the tissue. 



Accepting this conception of the different characters of the 

 different tissues, it will be seen that these may be divided into 

 two great groups, which we may term, provisionally, the lining 

 membrane group and the body pulp group. This division 



