THE STRUCTURK OF Till-: IIRANCHTK. 
9 
sections. Here and there is found a large granular wandering cell, markedly eosino- 
philous, and precisely similar in character to those seen in the connective tissue of the 
mantle and those scattered through the epidermis of both mantle and branchiae. These 
are to be regarded as granular leucocytes on their way to the exterior. They are shown 
in Figs. 9 and 13. 
The Blood Cells are normally colourless leucocytes, measuring on the average 
10 M in diameter (Fig. 11). - Some, however, are granular, and the granules may be 
yellow, brown, black, or green in colour. In diseased oysters, showing the green 
leucocytosis described below, the number of opaque granular leucocytes seems to become 
largely increased, and they are of a pale chalky-green colour. With osmic acid a black 
reaction is occasionally given by the granular leucocytes. In blood film preparations 
we have repeatedly tried to obtain evidence of an eosinophilous reaction in the normal 
leucocytes, but without success. On the other hand, the larger wandering cells in the 
tissues, or amoebocytes, are markedly eosinophilous. 
Eosinophilous Cells. — Although we have not observed eosinophilous leucocytes 
in the blood-vessels, there are numerous cells in the tissues which give an eosinophilous 
reaction. Large ovoid cells are met with, scattered with some regularity along the free 
edge of the gill filaments (PI. V., Fig. 3), and these, from the frequency of their 
occurrence, seem to be fixed elements of the epithelium (" Beckerzellen "). Cor- 
responding cells in the mantle (PI. V., Fig. 2) and intestinal epithelium have usually 
a pyriform appearance, and sometimes appear to be discharging their granular contents 
on the free surface. In the green American oysters described below, we have some- 
times observed that these cells give a well-marked reaction with pure dilute hasma- 
toxylin (PI. V., Fig. 8). There are also certain eosinophilous wandering cells which 
are found beneath the epithelium of the intestine, mantle, and branchiae, and which, in 
some cases, are seen making their way through the epithelium to the exterior (PI. V., 
Figs. 4 and 5) ; while other wandering cells — the green leucocytes of the American 
oyster — are never, in our experience, eosinophilous. The eosinophilous reaction, in fact, 
indicates merely a condition in which various epithelial and wandering cells ma>- be 
found. 
