466 GEORGE A. BAITSELL 



various cavities and open spaces of the embryo until after the 

 formation of the secreted ground substance which is the fore- 

 runner of the connective tissues. It appears evident therefore 

 that cells, whether in tissue cultures as shown by Harrison^^ 

 or in the developing embryo, have need of a supporting frame- 

 work of some kind in order for movement from the main masses 

 to take place. 



3. The connective tissue fibers begin to arise in the ground 

 substance soon after it has formed. In some cases, they form 

 in the ground substance in regions which are free from cells so 

 that it is certain that they have not arisen by an intracellular 

 action. The fibers are evidently formed by a gradual trans- 

 formation of the ground substance, first, into a delicate net-like 

 structure and then into the long fibers which are typical of con- 

 nective tissue. In its morphological features, at least, this 

 process gives evidence of being identical with the one previously 

 observed in the transformation of the plasma clot. 



4. In those regions of the ground substance into which the 

 cells have wandered, the formation of the fibers can also be shown 

 to be due to changes in the ground substance and not to a 

 sloughing off of the cell protoplasm. A differential stain, such 

 as ]\Iallory's connective tissue stain, shows definitely the bounda- 

 ries of the cell cytoplasm and of the ground substance. It is 

 suggested that the movements of the cells through the ground 

 substance introduce mechanical factors which aid in fiber forma- 

 tion just as has been shown to be the case in plasma clots. In 

 any case, the fact that the fibers can form in a cell-free ground 

 substance shows that no part of the cell cytoplasm is necessary 

 for their differentiation from the ground substance; nor is it 

 possible to demonstrate any morphological connection between 

 the cells and the fibers in the ground substance, 



19 Harrison, 1914, pp. 540 and 543. 



