26o THE BIOLOGY OF THE FROG chap. 



One important property of the white cells is their power 

 of ingulfing small bodies, which are taken in much as 

 an Amoeba takes in its food. Bacteria are devoured in 

 this way, and the leucocytes thus afford the body a meas- 

 ure of protection against these organisms, which are con- 

 stantly being introduced into the system in one way 

 or another, and might, if unchecked, be productive of 

 serious if not fatal effects. An irritation set up in any region 

 causes leucocytes to be attracted to the spot in large num- 

 bers. The introduction of bacteria into any part of the body 

 is followed by the invasion of that part by leucocytes, and it 

 frequently happens that the bacteria are devoured by these 

 cells before they gain a strong foothold. It is very probable 

 that one important factor which causes the movements of the 

 leucocytes in such cases is the presence of substances given 

 off by the bacteria which exercise a chemotactic effect upon 

 these wandering cells, causing them to congregate about the 

 center of diffusion. The chemotactic influence of such sub- 

 stances is shown by the following experiments first performed 

 by Massart. If a fine capillary tube sealed at one end be 

 filled with a culture fluid containing the bacterium Staphy- 

 lococcus pyogenes albus, and introduced into one of the large 

 lymphs spaces under the skin, it will be found in the course 

 of ten or twelve hours that a swarm of leucocytes have made 

 their entrance into the open end of the tube. If a similar 

 tube filled only with the culture medium be introduced, no 

 leucocytes will be found to enter it. The substances pro- 

 duced by the bacteria are apparently the cause of the inva- 

 sion of the tube by the wandering cells. 



Besides protecting the body against the germs of disease, 

 the white cells take part in the removal of tissue which has 

 become broken down and 'no longer of service to the organ- 

 ism. In the degeneration of the tail of the tadpole the 



