STUDIES ON THE PANCREAS OF THE GUINEA PIG 315 



of the same pieces of pancreas, and recounts of pieces which have 

 been teased a second time and remounted give variations in the 

 case of the resting pancreas of as high as 10 per cent. In the dis- 

 charged pancreas, on the other hand, where the error due to the 

 concealment of the smallest islets by the zymogen is eliminated, 

 the error is rarely more than five per cent. In making the counts, 

 overstained and understained pancreases were rejected. A cer- 

 tain degree of overstaining, however, is soon corrected after the 

 pieces are mounted under a cover, by the rapid reduction of the 

 dye in the acini. As table 2 indicates, in the pancreas of the new 

 born the islets are so abundant (as high as 338 in a single milli- 

 gram of pancreatic tissue), that accuracy is not so easily obtained 

 as in the older animals where the number in a milligram of tissue 

 rarely exceeds thirty-five. In these young pancreases the islets 

 are so close together that it is difficult to be sure either of not miss- 

 ing large numbers or of not counting large numbers of them twice. 

 Doubtless these two errors compensate one another to a certain 

 extent, but nevertheless it is probable that the error of counting 

 is greater in these cases than in the pancreas of an older animal. 

 The enormous variation in the number of islets at all ages makes 

 it difficult to interpret the facts with regard to such questions as 

 whether the number of islets increases with age. In the whole 

 series there were only three animals in which the number of islets 

 was greater than in a guinea pig two days old and weighing 74 

 grams. There are, however, certain facts which indicate that 

 during the first two weeks of life there is a reduction in the actual 

 numbers of islets, and that thereafter there is a slow production 

 of new islets. These facts will be discussed more fully elsewhere 

 in this paper, but it may be mentioned here that, in the new born 

 guinea pig, in addition to the islets consisting of two or more 

 cells, there are myriads of single islet cells located in the acini 

 and forming a part of the regular row of epithelium in these acini, 

 which do not enter into the counts because with the low powers of 

 the microscope necessary for counting they cannot be distin- 

 guished from connective tissue cells which contain large irregular 

 granules stained with neutral red. In the guinea pig one week 

 old, and at all times thereafter, such cells are rare although a few 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 3 



