l8o THE ENTODERMAL CANAL AND THE BODY CAVITIES 



tween the two ducts, and the duct of the ventral pancreas, plus the distal 

 segment of the dorsal duct, persists as the functional pancreatic duct 

 (of Wirsung) of the adult. The proximal portion of the dorsal pan- 

 creatic duct forms the accessory duct (of Santorini), which remains per- 

 vious, but becomes a tributary of the chief pancreatic duct. The ventral 

 pancreas forms part of the head and uncinate process of the adult gland. 

 The dorsal pancreas takes part in forming the head and uncinate process, 

 and comprises the whole of the body and tail. 



Accessory pancreatic duct 



Dorsal pancreatic duct ^Stomackj^ / Dorsal pancreas 



Dorsal pancreas 



, Ventral pancreas -^^i 



Ventral pancreatic duct lT\ Ventrd P a ^ 



Bile duct Bife duct Pancreatic duct 



FIG. 187. Two stages showing the development of the human pancreas: A, Embryo of 8 mm. ; 

 B, embryo of about 20 mm. (after Kollmann). 



In 10 mm. embryos the portal vein separates the two pancreatic anlages, and later 

 they partially surround the vein. The alveoli of the gland are developed from the ducts as 

 darkly staining celluar buds in fetuses of 40 to 55 mm. (C R). The islands characteristic 

 of the pancreas also bud from the ducts (and alveoli, Mironescu, 1910) and appear first 

 in the tail at 55 mm. (C R). 



Owing to the shift in the position of the stomach and duodenum during development, 

 the pancreas takes up a transverse position, its tail extending to the left. To its ventral 

 surface is attached the transverse mesocolon. 



Anomalies. The ventral pancreas may arise directly from the intestinal wall, and 

 paired ventral anlages also occur. Accessory pancreases are not uncommon. Both the 

 dorsal and ventral ducts persist in the horse and dog; in the sheep and man the ventral 

 duct becomes of chief importance; in the pig and ox the dorsal duct. 



THE BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



The Primitive Ccelom and Mesenteries. In the Peters embryo the 

 primary mesoderm has already split to form the extra-embryonic ccelom 

 (Fig. 74 C). When the intra-embryonic mesoderm differentiates, numer- 

 ous clefts appear on either side between the somatic and splanchnic layers 

 of mesoderm. These clefts coalesce in the cardiac region and form two 

 elongated pericardial cavities, lateral to the paired, tubular heart. Simi- 

 larly, right and left pleuro- peritoneal cavities are formed between the 

 mesoderm layers caudal to the heart. The paired pericardial cavities 



