514 The American Naturalist ` [June, 
even in such an apparently remote field as that concerning many 
diseases of the human race. 
In a tadpole, whose legs have commenced to grow, preparatory 
to assuming the adult condition, the tail begins to wither and dis- 
appear; and in this organ, Metschnikoff made another interesting 
and well-known discovery. The tissues were now of no further 
use to the organism, and he found that the leucocytes had attacked 
them, and were gradually eating them away. He often found 
unmistakable pieces of nerve and muscle-tissue inside their bodies, 
which were evidently undergoing a process of obliteration by 
digestion. In the pupa stage of insect metamorphosis the inter- 
nal organs are disintegrated, and here again the phagocytes attack 
the useless tissues and eat them. Certain strong, well-nourished 
cells, however, remain unharmed, and from these the organs of 
the adult are built up. 
Such an occurrence as this appears to be rather remarkable, 
and the question at once arises as to why these phagocytes 
should destroy one tissue, and apparently leave another unin- 
jured. Metschnikoff has conclusively shown that these wander- 
ing cells exert an undoubted choice in the selection of their food, 
and that they prefer dead to living tissue. His method of proving 
this fact was as follows. Taking some sea-urchin’s eggs, and kill- 
ing them by boiling, he carefully injected them under the skin of 
a nudibranch—one of the Gastropods,—and found that they 
were immediately surrounded by amceboid cells, and eaten by 
them in the usual manner. The same experiment was again 
tried, but this time the injected eggs were not killed. The result 
‘was that they not only remained unharmed, but, on the introduc- 
tion of spermatozoa among them, they were fertilized, and com- 
menced to develop. 
In such an experiment as this, it is noticeable that when a 
foreign body is introduced into an animal through an injured 
region of the skin, the leucocytes already in the vicinity are not 
the only ones which are ready to attack the invading particles. 
Very soon their fellows appear, having come from distant tissues, 
and take part in the fray; and not only do they attack organic 
substances, which by digestion would be assimilated, but also 
