2 1 6 E VOL UTION AND DISEASE. 



small, will cluster around and by a process of intracellular 

 digestion devour it. When the part to be removed is 

 large, the leucocytes will effect a separation between 

 it and the living body. Not only dead or damaged 

 portions of tissue are thus removed by leucocytes, but 

 useless parts, such as the tails and gills of tadpoles, 

 remains of larval organs, and the tails of ascidians, are 

 thus slowly removed. No animal tissue is capable of 

 resisting an attack of leucocytes. For instance, an ex- 

 amination of the milk-teeth of children or puppies at 

 the time they are shed, well attests the digestive powers 

 of these cells. Surprise is often expressed that when 

 such teeth tumble, or are dtelodged, from the gums 

 only the crown is present, the root, or fang, is usually 

 absent. That portion of the tooth in contact with the 

 gum is irregular, and an ordinary magnifying-glass shows 

 it to be full of bays and recesses. When such a tooth 

 is decalcified and suitably prepared for microscopical 

 scrutiny these bays are found filled with leucocytes 

 which, during life, were busily engaged in destroying the 

 fang of the tooth and have slowly induced its fall. The 

 shedding of milk-teeth in mammals, like the disappear- 

 ance of the tadpole's tail, is due to the persistent attacks 

 of leucocytes. Introduce into the tissues of a cat, a dog, 

 or a man, small pieces of clean sponge. In the course of 

 a few days the fragments of sponge will have disappeared. 

 Vary the experiment by removing the sponge two days 

 after its insertion, cut sections and examine under the 

 microscope, the interstices will be found occupied by an 

 army of leucocytes. Introduce some indigestible object 

 such as glass, a needle, or a fragment of metal. When 



