332 EMBRYOLOGY. 



(2) The Pancreas. 



The pancreas is developed in all Vertebrates with the exception, 

 of a few in which it is wanting (Bony Fishes) as an evagination on 

 the dorsal side of the duodenum, usually opposite to the origin of the 

 liver (figs. 162, 163, 186 p). In the Chick (fig. 186) the first funda- 

 ment is distinguishable as early as the fourth day ; in Man it appears 

 somewhat later than the primitive hepatic tube, and has been de- 

 monstrated by His in embryos 8 mm. long as a small evagination 

 (figs. 162 and 163). The sac, usually hollow, grows into the dorsal 

 mesentery (figs. 184, 188 p) by giving off hollow, branching, lateral 

 outgrowths. 



In the case of Man the pancreas is present as early as the sixth 

 week in the form of an elongated gland (fig. 164 p), the free end of 

 which has penetrated upward [cephalad] into the mesogastrium, 

 and thus, midway between the greater curvature of the stomach and 

 the vertebral column, it can move freely. It is therefore com- 

 pelled to share in the alteration of position which the stomach to- 

 gether with its mesentery undergoes. In embryos of the sixth week 

 its long axis still corresponds approximately with the longitudinal 

 axis of the body. The free end then moves into the left half of 

 the body-cavity, the whole organ being turned (fig. 166) until finally 

 its long axis comes to lie in the transverse axis of the body, as in the 

 adult. In this position its head is imbedded in the horseshoe-shaped 

 curvature of the duodenum, whereas its tail reaches to the spleen and 

 left kidney. 



Inasmuch as the pancreas in its development has grown into the 

 mesogastrium (figs. 164, 166, 188), it possesses in the first half 

 of embryonic life, as TOLDT has shown, a mesentery, on which it 

 accomplishes the turning previously described. But at the fifth 

 month this disappears. (Compare the diagrams fig. 167 A and B p.) 

 For as soon as the gland has taken its transverse position, it at- 

 taches itself firmly to the posterior wall of the trunk and soon loses its 

 freedom of motion, because its peritoneal covering and its mesentery 

 become fused with the adjacent parts of the peritoneum (fig. 167 

 B gn 4 ). In this manner the pancreas of Man, which was developed^ 

 like the liver, as an intraperitoneal organ, has become a so-called 

 extraperitoneal organ, owing to a process of fusion between the 

 serous surfaces that come in contact with each other. By means of 

 this also the attachment of the mesogastrium is displaced from the 

 vertebral column farther to the left. 



