60 



kind seems to be gradually transformed into another, and 

 probably there is only one type of secreting cell present. 



The ducts (fig. 50) conveying the secretion to the 

 stomach divide up in the gland into several branches lined 

 with ciliated epithelium. The cells are columnar and 

 granular in appearance, with a prominent nucleus near 

 the base. They are supported by a layer of connective 

 tissue — the tunica jjvoiJfia (fig, 50, Tu. 'p.). Outside 

 this, again, is a layer of circular muscle fibres, which pass 

 round Llie duct forming the tunica muscvlaris (fig. 50, 

 Tti. tn.) " Macroblasts," or eosinophilous cells, as seen in 

 (he oyster, are not present in Pecten, though under the 

 action of the same fixatives and stains they show up 

 strongly in the mantle. 



The only type of cell present appears to be the 

 granular cell lining the alveoli (fig. 50, -4/.), but smaller 

 cells can be observed between some of these, which stain 

 much more intensely than the others. These are probably 

 similar to the cells described by MacMunn (21) in the 

 gastric gland of Patella, and are young cells which will 

 replace the others, for, in sections through the alveoli, the 

 granular cells can be seen in process of being shed into 

 the lumen, and there are bodies in the course of the ducts, 

 of common occurrence, that are undoubtedly these shed 

 cells on their way to the intestine. In this way the 

 digestive gland appears to have an excretory function 

 in addition to its storing and pancreatic functions. 

 Pigment concretions do not appear in the lumen of any 

 of the alveoli. Cilia are confined to the ducts, and as an 

 alveolus is followed towards its caecal end the cells become 

 fewer and more swollen (fig. 5, Al.') and intensely 

 vacuolated, the nviclei lying against the connective tissue 

 membrane supporting the cells. Eventually the lumen of 

 the alveolus disappears altogether, and the cells meet in 



