CYTOMORPHOSIS. 31 



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. Differentiation 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 em- 

 bryonic life all the cells become specialized, and with the acquisition of specializa- 

 tion 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 

 undifferentiated 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 

 destructive. 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, hypertrophic degeneration or indirect cell death preceded by 

 growth and structural change of the cell, often with nuclear proliferation. Di- 

 rect cell death implies that the cell loses its vitality, and, being dead, disin- 

 tegrates; 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 occurrence 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. 



Correct notions on this subject are so important that a few illustrations may 

 be mentioned. Let us begin with necrobiosis. There are organs whose exist- 



*With possibly very rare exceptions. 



