64 Swale Vincent and F. D. Thompson, 



characteristic of the latter. Their edges appear under a low power 

 to be for the most part clearly defined, though there is no trace of 

 any kind of capsule surrounding them. 



The islets consist of solid masses or of branching columns of cells, 

 with interspaces occupied by a relatively wide capillary network. The 

 protoplasm of tlie cells composing these solid columns or masses is 

 only clear by contrast with that of the zymogenous cells. In reality, 

 as may be seen under a high power, it is very finely granular. One 

 of the most striking features, even in a casual observation of an islet 

 is the large number and prominence of the nuclei.^) In a given area 

 of the field very many more of them are visible than in a corre- 

 sponding area of zymogenous tissue, in the average proportion of about 

 25 to 6 (see PI. IV. fig. 1). It follows that the cells composing the 

 islet are smaller than those of the secreting alveoli (PL IV, fig. 1; cf. 

 lept. c. with zijm). 



The actual form of the solid masses of cells constituting the islets 

 varies in different cases. In some of them the arrangement is very 

 much what we might suppose would be brought about if the alveoH 

 in certain definite regions became narrowed in diameter, each cell 

 becoming much smaller, the nuclei brought closer together, the lumen 

 closed, and the zymogenous granules eliminated. This appearance is 

 well shown in PL IV, fig. 1 (z., lept. c). 



In other cases, especially in the smaller islets, this arrangement 

 is not so obvious (PL IV, fig. 2). 



Frequently a distinctly alveolar arrangement is indicated by a 

 double row of nuclei in the columns of cells, suggesting very sti-ongly 

 that in the constituent columns of the islets we have simply to deal 

 with modified secreting tubules (PL IV, fig. 1). 



The cells of the islet are, as previously stated, smaller than those 

 of the secreting tubules, irregularly cubical or polyhedral in shape, 

 and of an average greatest diameter of 12 u. The nuclei have an 

 average diameter of f) /< as against 7 or 8 // in the secreting alveoli. 



') 'I'lic. cuiispiciioiisiicss of the iiiulci is of coiirsi'. in |Piii'l diic lo (lit' absence 

 of zj iiKij'fiiuiis ^'f.'iiiiili" . Iiiil cliii'lly tu llirir imi'cii' 1(1 iiiiiiiIht in ;( fivcn .•ii'oa. 



