CYTOMORPHOSIS. 29 



each member of which has its predestined history. It is fortunate for our com- 

 prehension of embryological processes that we are already able to say that Roux's 

 hypothesis is erroneous. 



We must start, then, with the right conception of the ovum, every part of 

 the protoplasm of which is to be regarded as potentially capable of producing 

 any or all of the tissues of the adult. 



2. Differentiation. — This may be defined as a process by which the structure 

 of the cells is modified, so that cells become dissimilar in structure by acquiring 

 an organization which adapts them to special functions. The cells which arise 

 during the segmentation of the ovum differ but slightly from one another. As 

 development progresses we find the cells change, some in one way, some in aii- 

 other, so that many kinds of cells are produced, but of each kind we find a large 

 number of cells. Each kind of cell may be said, roughly speaking, to form a 

 tissue for itself. Cells of each tissue offer visible peculiarities by which they may 

 be readily distinguished from one another under the microscope. It thus ap- 

 pears that the production of tissues is the main result of differentiation, so that 

 this process of development may be fairly accurately defined as equivalent to 

 histogenesis. As to the factors which cause differentiation, we have no satis- 

 factory knowledge. We can, at present, only note the changes, when they 

 acquire such magnitude as to become microscopically visible. As to the phys- 

 iological conditions which cause these changes we have almost no conceptions. 

 It is probable that the nucleus has a leading role to play, but our knowledge of 

 this role is too little advanced to permit a profitable discussion of the subject 

 here. 



The actual process of differentiation shows itself both in the protoplasm 

 and in the nucleus of the cell. The changes in the latter are the more conspicu- 

 ous, and therefore the better known. The changes in the nucleus have still to 

 be adequately studied. The changes in the protoplasm are twofold: First, in 

 the intimate structure of the protoplasm itself and in the size and disposition 

 of its strands and filaments; secondly, in the character of the various substances 

 to be found imbedded in the protoplasm. These two kinds of change are well 

 illustrated, the first, by the nerve-cells; the second, by the gland cells, for in- 

 stance, in the pancreas. The student can easily see that the character of the 

 protoplasm in the adult nerve-cell differs profoundly from that of a cell from one 

 of the embryonic germ-layers, and that the body of the nerve-cell consists of 

 protoplasm with little, if any, of other substances imbedded in it. In the se- 

 cretory cells of the pancreas the zymogen granules are conspicuous; their dis- 

 tribution, uniform size, and refractile qualities demonstrate immediately their 

 unlikeness to anything found in the embryonic cells. These granules are not 



