210 GEORGE W. CORNER 



lary bed of a terminal arteriole measures the unit. The villus 

 of the intestine is supplied by a terminal arteriole; and instances 

 might be multiplied to show that it is the circulatory system which 

 in this way lays off the organ into units. Moreover, the division 

 is one of almost mathematical exactness. The investigations of 

 C. Ludwig and his pupils have shown that all the capillaries con- 

 necting the terminal tips of the arteries and veins are of the same 

 length in any one organ. Then Thoma (76) has made clear that 

 all the capillaries of any one organ are equally favored with re- 

 spect to pressure and rate of flow by their very laws of growth; 

 and Mall, as we have seen, extended this idea by showing that 

 all the units of an organ (the liver) are equally favored with 

 respect to the circulation. 



To summarize briefly: Those subdivisions of glandular or- 

 gans which we discover by the eye, dissection and the micro- 

 scope, are not random; these organs are composed of regularly 

 repeated units, each complete in itseh as to structure and func- 

 tion, whose size is determined by the length of blood-capillaries. 

 Such units appear to persist thoughout the development and 

 adult state of the organ; and if for some special reason the unit 

 is concealed by over- or under-growth of the limiting connective 

 tissue at one stage of ontogeny, it may still be traced at another. 



We have seen that there is much confusion regarding the sub- 

 divisions of the pancreas and their significance. The following 

 study is undertaken, therefore, to determine the structural unit 

 and method of gi'owth of the pancreas by careful investigation 

 of a single species, in the light of the general conception outlined 

 in the foregoing pages, and to make whatever addition may be 

 possible to the general theory. 



TECHNIQUE 



The pancreas of the pig was taken as the object for study be- 

 cause of the ease of obtaining embryonic material in all stages, 

 as well as adult. As the new observations reported in this 

 paper, as far as the development is concerned, depend upon a 

 method of injecting the ducts of the embryonic and foetal pan- 

 creas, I shall describe the technique in some detail. 



I 



