CYTOMORPHOSIS 25 



observe no similar developmental processes in unicellular 

 organisms. The transformation of cells which leads to the 

 formation of tissues is designated, as differentiation. In the 

 earliest stages of the embryo the cells are remarkably like 

 one another, Fig. 15, but in the course of their further develop- 

 ment they become unlike or different; hence the designation 

 differentiation. How these differentiations arise is an ex- 

 tremely interesting question about which we know very 

 little, because as yet we have become acquainted almost 



FIG. 15. Section through the posterior part of a rabbit embryo of seven and 

 a half days, to show the three germ layers, each of which consists of undifferentiated 

 cells. Magnification 250. 



exclusively only with such alterations as are visible with the 

 microscope. The visible alterations, however, we must 

 assume, are the consequence of chemical processes which we 

 still have to discover. The visible alterations have been 

 studied with the utmost care by many eminent biologists, 

 and we are able to say that they follow strict laws. It is 

 convenient to have for the complete transformation of cells a 

 short, scientific term. As such I propose "cytomofphvsis" 

 We are now to occupy ourselves with the laws of cytomor- 

 phosis so far as these have been determined. The develop- 

 ment of simple cells into differentiated we call progressive 



