212 GEORGE W. CORNER 



main. The duct is seen as a translucent streak passing down to 

 the intestine about in the center of the duodenal portion of the 

 head (figs. 12, 13). When it is located the needle is thrust into 

 it just where it emerges from the intestinal wall, and the ink 

 is blown in through a rubber tube held in the operator's mouth. 

 The progress of the injection can be watched and controlled, 

 but the method requires much more dexterity than the other. 

 In foetuses from 70 to 80 mm. the operation is quite a difficult 

 one, for the duct is very small, not easily distinguished, and 

 yields before the needle. With practice, however, a complete 

 series of preparations can be obtained by these two methods. 

 The specimens are preserved in 10 per cent formalin, dehydrated, 

 and cleared by Spalteholz's or other clearing fluids. 



The blood-vessels are filled with ink, Berlin blue, or silver 

 nitrate solution through the aorta or coeliac axis, according to 

 the size of the embryo. Double injections of embryonic ducts 

 and blood-vessels are easily made, but prove confusing through 

 complexity. I have used the ordinary simple methods of inject- 

 ing the adult specimens, the tissue being cleared en masse or 

 cut into serial sections. 



PANCREAS OF 100 TO 200 MAI. PIG 



The best point at which to begin the study of the structural 

 unit of the pancreas is in the foetus of 100 to 200 mm. length 

 (figs. 1,2). At this stage the organ is relatively small and simple, 

 and the duct and blood-vascular systems are injected without 

 difficulty. In form and relations the gland is closely similar to 

 that of the adult. At the 120 mm. stage the head lies close to 

 the duodenum for a distance of 4 or 5 mm.; the duct enters the 

 duodenum at the lowest point of the head. The tail stretches 

 toward the spleen, and is joined to the head by two narrow 

 bridges of gland tissue, the vipper one of which (fig. 1, UB) rep- 

 resents the dorsal anlage, the lower (fig. 1, LB) the ventral an- 

 lage, according to Thyng ('08) (fig. 2, B). Wliere the lower 

 bridge joins the tail there is a broad thick descending process 

 which fits neatly into the upward curve of the duodenum (fig. 1, 

 Pr.). Minor variations from this form will be seen in many of 



