196 PHOTO-MICROGRAPHS. 
An excessive development of the cells of a 
part or an infiltration of foreign cells among the 
normal histological elements results in the form- 
ation of a tumor—non-malignant, as in fatty 
tumors, etc., or malignant, as in the varieties 
of cancer. 
We have an example of this in Plate XVII. 
Fig. 2, in which spherical nucleated cells are to 
be seen infiltrated among the tissues (the female 
breast), constituting the malignant disease known 
as sarcoma, a variety of cancer. 
Parasires. PrLates XVIII. anp XIX. 
Microscopists have given much time and re- 
search to the study of the parasitic fauna and flora 
of plants and animals. But the field is so broad 
that there is still much to be learned in this direc- 
tion. The larger parasites, such as are seen in our 
Plates XVIII. and XIX., have long been known, 
and, indeed, some of them may be readily seen 
with the naked eye. 
These more conspicuous animal parasites be- 
longing to the class Insecta (Plate XVIII. Figs. 1 
and 2) and to the class Arachnida, family Acarina 
(Plate XIX. Figs. 2 and 3), although they furnish 
a large number of species, nearly every bird and 
mammal being infested with one or more species 
peculiar to it, have less practical interest for ciidized 
man than have the Entozoa (internal parasites) 
and the parasitic Fungi, against the attacks of 
which he finds it more difficult to defend himself. 
