216 EVOLUTION 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 dislodged, 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 
