692 E. V. COWDEY 



lated cells become fewer and fewer, but a few persist, even in the oldest 

 animals examined. On the whole, the islands seem to be rather more 

 abundant in young animals. Throughout life there is a slow production 

 of new islands from undifferentiated duct epithelium. The claims of cer- 

 tain investigators to the contrary, that island tissue is developed from the 

 epithelium of the secreting acini no longer merit consideration. 



Growth. The comparative growth energy of the island tissue has not 

 been measured by the inanition method. That is to say, we do not know 

 whether the islands of Langerhans should be classified with those endocrin 

 organs, like the parathyroids, which continue to grow in young animals 

 held at maintenance of body weight by underfeeding. 



Comparative Anatomy 



Islands of Langerhans, or structures closely resembling them, occur in 

 all vertebrates, from fishes to man, though they differ considerably in 

 relative size and in number. It is interesting to note that in fishes the 

 island tissue is condensed into a single large mass, quite unconnected with 

 acinus tissue (Rennie), a condition which affords a unique opportunity 

 for experimental study. Dewitt has investigated the arrangement of the 

 islands in quite a comprehensive series of phyloge-netic stages. 



With the improved methods of enumeration at our disposal, we may 

 confidently expect that studies on the relative amounts of island tissue in 

 different forms will yield interesting results, because the varying demand 

 for secretion may be correlated directly with some comparatively well 

 known attribute of the organisms concerned. In this way it may be 

 possible to secure a definite clue to the function of the islands. 



Histology 



Histologic Morphology. The cells of the islands of Langerhans do 

 not constitute a syncytium, as was formerly supposed. Their outlines are 

 discrete and can easily be distinguished in suitable preparations. The 

 cells are often arranged in cords with sinusoids taking the place of the 

 regular ducts ; for the lumina of the ductules, with which the islands are 

 in contact, rarely penetrate into their interior. In the case of the larger 

 islands, the cells are bound together by a very light connective tissue 

 framework. In the smaller, on the other hand, it is quite common to find 

 that the island cells are in direct continuity with the acinus tissue. 



Cytology. In ordinary preparations the cells of the islands all look 

 alike (Fig. 3), but after special methods of fixation and staining which 

 we owe primarily to Bensley, it is possible to see that marked differences 



