3O CYTOMORPHOSIS 



its cells are simple and more or less similar to one another.^ 

 Tissues in the stricter sense are not present. In spite of the 

 fact that these cells attain their further development under 

 unnatural conditions they in themselves form muscle fibers, 

 connective tissue and bone. In spite of the fact that the 

 microscope shows us nothing in these cells by which we can 

 recognize their future development, we must assume that 

 the specification already exists. Professor Harrison, as I 

 have already mentioned, devised a method to cultivate tissues 

 in vitro. One can cut out from an embryo chick little pieces 

 at will and cultivate them artifically in vitro and bring them 

 to further development. In this manner W. H. Lewis has 

 succeeded in studying the specific cell formation. The 

 cells of the mesenchyma grow in the manner of mesenchyma ; 

 the cells of epithelium as epithelium. Neither in the nucleus 

 nor in the protoplasm in these cells can we demonstrate 

 peculiarities which we can regard as the causes of the unlike- 

 ness of their growth, but surely there exist in these cells 

 peculiarities which are not visible to us and which determine 

 the performances of the cells. It is not going too far to 

 assume that in all cases the invisible alterations of protoplasm 

 precede the visible. 



The young cells in an undifferentiated vertebrate embryo 

 have little protoplasm. The first thing that must happen is 

 that the protoplasm grows, a phenomenon which one may 

 easily observe with the microscope. After the protoplasm 

 has grown, differentiation proper may begin. It is always 

 gradual and consists essentially in this, that something new 

 becomes visible in the protoplasm. In part, especially in the 

 so-called epithelium, we have to do with the formation of 

 superficial membranes around each cell. More important 

 probably are the new formations in the protoplasm, Fig. 18. 



