318 R. R. BENSLEY 



the relation of the islets to the ducts. Here it will suffice to state 

 that in accordance with the observations of Helly ('05), Weich- 

 selbaum and Kyrle ('09) for the embryonic pancreas and for early 

 life, islets in all stages of formation may be found in connection 

 with the ducts of the pancreas of the guinea pig throughout life. 



The most striking feature of table 1 is the great difference be- 

 tween the actual number of islets and that determined by previous 

 investigators by the method of counting in sections. Dewitt 

 ('06) estimated the number of islets in the pancreas of the guinea 

 pig as 1.14 per cubic millimeter. If we multiply the number per 

 milligram shown in my tables, by the specific gravity of the pan- 

 creas we obtain the real number of islets per cubic millimeter. 

 The minimum content found by this method is 9.9 per cubic 

 millimeter, the maximum 197.9 per cubic millimeter. The aver- 

 age content of the whole pancreas for guinea pigs between 300 

 grams and 600 grams weight is 22.28 islets per cubic millimeter, 

 which is 19.5 times the number estimated by Dewitt. 



The reason for this discrepancy is obvious enough when one 

 compares the neutral red preparations with sections of the pan- 

 creas. In the former every islet stands out clearly defined what- 

 ever may be its size, but in the latter many of the small islets 

 require study under oil immersion lenses to distinguish them from 

 accumulations of centroacinous cells or small collapsed ducts, 

 for it is only under the high power that the specific contents of 

 the islet cell can be well seen. Accordingly by the section method 

 of enumeration, practically all of the smaller islets escape the 

 count. 



The difference between the real number of islets and the appar- 

 ent number of them as seen under the low power in sections ac- 

 quires an important significance when the latter method is applied 

 to experimental investigation. For what reliance can we place 

 on a method of investigation in which the minimum error is 88 

 per cent of the real number, and the average error 94 per cent? 



Table 2 shows the distribution of the islets of Langerhans in 

 the pancreas of guinea pigs in terms of the number of islets in 

 1 mg. of pancreas. The results are obtained by counting the 

 pieces separately, and then weighing them as described under 



