266 ON GROWTH AND OVERGROWTH 



around them lead to the deposit of a calcareous or bony matrix 

 and gradually assume all the characters of typical bone corpuscles. 



From the variability of the structure of the glandular organs 

 it is difficult to lay down any general rule with regard to re- 

 generation of glandular epithelium, so that it must be confessed 

 that, partly on account of this variability, partly on account of 

 the fact that the different observers upon glandular regeneration 

 have not had this conception of mother cells before them in their 

 studies, such glandular epithelium in the present state of our 

 knowledge affords the least satisfactory demonstration of the 

 principle. There are, however, so many instances concerning 

 which we have positive information as to render it more than 

 probable that throughout the same principle is in action. 



Of the mother cells of the type seen in the Malpighian layer 

 of the epidermis, several instances may be called to mind : the 

 mother cells of the testicular epithelium and (though differing 

 somewhat in plan) of the ovarian follicles, of the sebaceous and, 

 apparently, from Tornier's studies, the mammary glands. In 

 both of the latter organs there is, during activity, a giving off of 

 cells, and mitosis occurs especially in the deeper, more external 

 layers. 



In this connexion should, I think, be mentioned the columnar 

 and ciliated epithelia of mucous membranes. It is remarkable 

 how one histological text-book after another figures such columnar 

 epithelium as formed of a palisade of fully developed columnar 

 cells in regular series situated upon the basement membrane, 

 without indicating or even referring to the presence of embryonal 

 mother cells lying between the bases of the fully differentiated 

 columnar cells. Certainly under pathological conditions one is 

 impressed by the fact that these basal cells are present, and that 

 there is a development of new cells from beneath to take the 

 place of the degenerating fully formed cells. While in the simpler 

 acinous glands it is difficult to recognize such permanent mother 

 cells (indeed they are probably absent in the adult), here at 

 least they are present. 



In the lymph glands we may have another arrangement. In 

 them the mother cells are collected together in the form of small 

 nodes in the centres of the individual follicles ; a somewhat 

 similar arrangement of foci of embryonal cells is met with in the 

 thyroid, while in the pancreas, Langerhans's bodies or " cell 



