DEVELOPMENT OF SUBOCULAR LYMPH SACS 29 



of the infraorbital vein (23), in which region it is seen that the 

 subocular sac has not yet made its appearance. The embryo 

 from which figure 25 was taken is represented in reconstructions 

 by figures 1 and 2. 



In view of the observations thus far made on the living embryo 

 and by means of sections, it may be stated that: (1) The develop- 

 ment of the anlagen of the subocular lymph sacs of the trout is 

 initiated soon after a definite haemal circulation has been estab- 

 lished. The lymph present in the intercellular tissue spaces of 

 the head region is then directed toward these anlagen which are 

 uniformly situated at definite focal points within an otherwise 

 densely arranged mesenchyme; (2) the gradual increase in the 

 amount of lymph received by the subocular lymph sacs during 

 the stage of their independence, results in the application of a 

 constant and continuous pressure to the mesenchymal cells form- 

 ing their walls, which in itself must be a positive factor in causing 

 these cells to flatten out and gradually to assume an endothelial 

 form. In fact, the recent experimental investigations of W. C. 

 Clarke ( ? 14) appear to bear out this view in a most decisive 

 manner. Clarke's observations are as follows: 



Following a purely physical injury which destroys the free surface 

 cells of the peritoneum, pleura, or the lining cells of blood vessels, two 

 possibilities exist as to how regeneration of the damaged zone pro- 

 ceeds. (1) Cells grow from the periphery of the given denuded area, 

 taking origin from the adjacent, previously existing and intact flat 

 surface cells; (2) the exposed deep connective tissue cells making up 

 the floor of the injured area proliferate, change in form, becoming 

 flattened. 



These experiments were undertaken in reference to the latter possi- 

 bility in the regeneration of surface cells to learn what happens as 

 regards connective tissue cells in contact with a smooth surface, whether 

 solid or fluid; in other words to learn what change in form takes place 

 in the investing connective tissue cells in contact with the surface of a 

 non-irritating body, placed for a time in the subcutaneous tissue of a 

 living animal. 



The sections showed that the cells in contact with the surface of the 

 foreign bodies were changed in form in all of the specimens into large, 

 flat cells placed edge to edge resulting in a definite sheet. The silver 

 salt demonstrated a mosaic of black silvered lines marking the cell 

 outlines of the lining of surface cells. 



