232 GEORGE W. CORNER 



The question arises, how do the few units of the embryo grow 

 into the thousands of the adult, and what determines that the 

 unit shall have a definite size? Laguesse has found that in the 

 sheep the proliferation of cells in the growing embryonic pan- 

 creas takes place all along the acinar wall, but chiefly at the dis- 

 tal ends of the acini. Therefore, the growth must be most 

 marked at the periphery of the unit, since the greatest number 

 of acini terminate here. As I have said, the units in pig embryos 

 are of all shapes. A small proportion of them are spherical or 

 ovoid, but most are lobulated by depressions of the surface into 

 from two to five parts. The lobulation may be very slight, or 

 it may have gone so far that it is a question whether to call the 

 bit of tissue one unit, or several (fig. 17, A, B). Whenever the 

 lobulation is marked, we find a tiny branch of the artery running 

 into each portion of the unit ; the increased tissue area has caused 

 a freer circulation, and the vascular system has responded by 

 transforming a capillary into an arteriole. If the growth con- 

 tinues, our slender arteriole will also grow to supply a capillary 

 net as large as the parent unit; the branch duct which ran to 

 that part of the original unit has become a unit duct; and in 

 this way are formed two, three, four, or five units from one. In 

 the tail of the organ, where the growth in length is to be great, 

 the units generally divide at first into two, end to end, so as to 

 run out long chains of units (fig. 18). In this way rapid exten- 

 sion is gained. At the head, however, density of growth is rather 

 to be obtained, and here the multiplication of units is from the 

 first by fours and fives (figs. 19, 1). Since a spherical form gives 

 the largest volume for a given radius, we find the imits rounded 

 as long as there is room for them without compression. 



It is very plain that the size of the pancreatic unit is dependent 

 upon the vascular system; the unit contains as much tissue as 

 can be nourished by the capillary bed of one arteriole. In the 

 eml^ryo, in the space between neighboring capillary tufts there 

 is less circulation, and hence the connective tissue, which de- 

 mands less subsistence than the gland-cells, fills in the space 

 between the capillary tufts. In the embryo the units stand 

 clearly apart, but as the pancreas grows in bulk the tissue becomes 



