THE STRUCTURE OF THE BRANCHIAE. 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 hsema- 

 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 may be 

 found. 



