STUDIES ON THE PANCREAS OF THE GUINEA PIG 303 



mounted in salt solution under cover glasses, without pressure 

 (pressure easily disintegrates the larger islets and disperses their 

 cells among the acini), and counted. The boundaries of the 

 lobules serve as a guide to prevent counting islets twice, and with 

 a little practice a fair degree of accuracy can be attained. After 

 all the islets are counted the pieces of pancreas are again collected 

 from the slides, keeping separate the pieces belonging to the major 

 divisions of the pancreas, and after pressing lightly between lay- 

 ers of filter paper to remove excess salt solution, are placed in 

 weighing tubes and weighed. Thus a total count of the islets, 

 and weights are obtained, which enable one to determine with a 

 fair degree of accuracy the ratio of islets to pancreas, and to body 

 weight. 



The chief sources of error in this method are due, first, to over- 

 staining of the pancreas, when the zymogen granules stain slightly, 

 and so obscure the smaller islets, thus making the resting pancreas 

 more difficult to count accurately than the active pancreas where 

 there are few zymogen granules; second, to the high refractive 

 index of the zymogen granules making the pieces less transparent ; 

 third, to careless mounting of the pieces, so that they are not 

 sufficiently transparent; and lastly, to the error in comparative 

 weighing, owing to different amounts of salt solution being 

 abstracted from the pieces in different cases. The latter error 

 might be considerable if several different workers were preparing 

 the material for weighing, but in the hands of a single worker 

 following a definite routine, the results should be fairly accurate. 

 The neutral red method has been successful for demonstration 

 of the islets in all pancreases examined, which include the follow- 

 ing: Cavia, Lepus, Mus, Felis, Canis, Sus, among mammals; 

 Anolis, Chelydra, among reptilia, Rana, Bufo, and Necturus, 

 among amphibia; and Columba among birds. As a method for 

 enumeration it is most successful in the rodents, less so in the cat 

 and dog, for the reason that in the latter species the zymogen 

 granules take up more of the neutral red, and on the contrary 

 the cells of the islets, containing relatively fewer granules, stain 

 less intensely than those of the rodent species. For these animals 

 the next method is preferable. 



