G94 



E. V. COWDRY 



l^t 



exist (Fig. 4). At least three different varieties may be distinguished 

 in the guinea pig. 



B Cells. The B cells are by far the most abundant and constitute 

 the bulk of the tissue (Fig. 4). They contain a large number of tiny 

 granules, closely packed together, which, after fixation in the acetic-osmic- 

 bichromate mixture, are basophilic, staining with methyl green. They 

 also stain blue with neutral gentian, contrasting sharply with purple 

 zymogen granules, Mitochondria occur, scattered throughout the cyto- 

 plasm, and sometimes show a tendency to be heaped up in the region of 

 the cell adjacent to the sinusoid. The reticular apparatus is usually con- 

 fined to the opposite pole of the cell, 

 remote from the sinsoid, indicating the 

 possibility of the existence of some 

 measure of secretory polarity. The nu- 

 clei are roughly spherical and highly 

 chromatic. Mitotic figures are never 

 seen. Since the B cells are the only 

 ones which it has been possible to alter 

 in structure experimentally, we natur- 

 ally suspect that they are responsible 

 for the physiologic activity of the 

 islands. 



A Cells. The A cells are much 

 fewer in number, to the point of being 

 entirely absent in some islands, and they 

 contain -quite different granules, which 

 stain with fiiclisin, instead of with 

 methyl green, after acetic-osmic-bichro- 

 mate fixation. These cells may also be 

 identified in neutral gentian prepara- 

 tions, owing to the fact that they are colored somewhat diffusely 

 (Fig. 4). The solubilities of the granules are likewise distinctive. 

 The reticular apparatus is much the same, but the nuclei are oval and 

 comparatively poor in chromatin. Mitotic figures are absent and there is 

 every reason to believe that these cells are as highly differentiated as are 

 the B cells. 



Indifferent Cells. The "indifferent" cells are the rarest (Fig. 4). 

 They are devoid of specific granulations, and appear quite clear in the 

 sections. Only the reticular apparatus and the mitochondria, in addition 

 to their nuclei, can be distinguished. They are apparently identical with 

 the cells of the ductules, which may often be seen dividing. According 

 to Bensley, we may regard them as the parent tissue, from which the A 

 and B cells arise. 



These three cell types are fundamental, since they occur in man, 



Fig. 5. Island of Lan^erhans in 

 the pancreas prepared by Cajal's Ura- 

 nium Nitrate Silver method which 

 blackens the reticular apparatus. 

 After Cajal. 



