The Behavior of Cells 191 



at the seat of injury. The aggregation of these 

 wandering cells may actually be observed under the 

 microscope in the transparent living mesentery of 

 the frog. If a part of the mesentery is drawn out 

 of the living animal and stretched over the stage 

 of a microscope the blood may be observed stream- 

 ing through the capillaries, and the individual cor- 

 puscles distinctly followed in their course. If a re- 

 gion near a capillary is pricked with a hot needle 

 the white corpuscles may be seen to pause in their 

 course as they arrive near the injured area and 

 pass through the capillary wall. Here again it is 

 probably some chemotactic proclivity that causes 

 the leucocytes to congregate. Combined with the 

 power of these cells to devour bacteria this chemo- 

 tactic tendency enables the leucocytes to play the 

 part of watchful protectors in checking infections 

 and destroying products of decay. 



Besides their role in maintaining the normal ac- 

 tivities of the body the leucocytes often play an 

 important part in development. In the formation 

 of an organism the tearing down of previously es- 

 tablished structures is often a necessary preliminary 

 to further advance. The resorption of the tail of 

 the tadpole is a process of involution in which the 

 leucocytes play an essential role in destroying the 

 fragments of the degenerating structure. The or- 

 gans of the caterpillar are very largely destroyed 

 in the quiescent pupa state in which the reorganiza- 

 tion of the body occurs that results in the forma- 



