CYTOMORPHOSIS. 15 



entiation, and another portion remains more or less undifferentiated and retains 

 more or less fully the power of continued proliferation. The epidermis is a good 

 representative of this type. Its basal layer consists of embryonic cells, which 

 multiply; some of these cells move into the upper layers, enlarge, and differentiate 

 themselves into horny cells; others remain in the basal layer and continue to mul- 

 tiply. The progeny of a given basal epidermal cell do not all have the same fate, 

 but divide themselves into two kinds of cells, one kind retaining the ancestral 

 character, the other becoming something new and unlike the parent cell. Differ- 

 entiation according to the second type is characterized by its inclusion of all the 

 cells. This type has its culminating and most perfect illustration in the central 

 nervous system, where comparatively early in embryonic life all the cells become 

 specialized, and with the acquisition of specialization they forfeit their power of 

 multiplication the neuroglia cells partly, the nerve-cells wholly.* The growth of 

 the brain after early stages depends not on the proliferation of cells, but chiefly 

 upon the increase in size of the individual cell. The correctness of this statement 

 is not affected, in my belief, by the fact that epithelial portions of the medullary 

 tube in comparatively late stages may be added to the nervous portion, the cells 

 multiplying rapidly, as we see at the growing edge of the young cerebellum. The 

 brain here grows by the addition of cells in the indifferent stage, but as soon as 

 these cells are differentiated they conform to the general law and divide no more 

 (neurones) or slowly (glia cells). 



The importance to pathologists of a thorough knowledge of the genesis of the 

 tissues from their germ-layers can hardly be emphasized too strongly, for it is more 

 than probable that all pathological tissues are as strictly governed by the law of 

 genetic restriction as are the normal tissues. 



3. Regression. The use of this term does not imply that a cell can move 

 backward after differentiation into a stage of lower differentiation or into an un- 

 differentiated condition. So far as we know at present, such a change does not 

 occur, and we therefore look upon it as impossible. Regressive changes are very 

 unlike the constructive changes which appear in differentiation, for they are destruct- 

 ive. They fall into three main groups: first, changes of direct cell death; second, 

 necrobiosis or indirect cell death preceded by changes in cell structure; third, hyper- 

 trophic degeneration or indirect cell death preceded by growth and structural 

 change of the cell, often with nuclear proliferation. Direct cell death implies that 

 the cell loses its vitality, and, being dead, disintegrates; or, may be, is removed by 

 some means, chemical or phagocytic, before disintegration occurs. Necrobiosis and 

 hypertrophic degeneration are normal processes, which invariably occur in the 

 normal body and play an important role in its development. Without their occur- 

 rence on a large scale the normal round of human life would be impossible. The 

 student should free himself from the unfortunate tradition that these processes are 

 exclusively pathological. 



* With possibly very rare exceptions. 



