BODY CAVITIES, DIAPHKAGM AND MESENTERIES 1 79 



same time, growth and bending of the bile duct to the right bring the ventral 

 pancreas into close proximity with the dorsal pancreas (Figs. 185 and 187). 



In embryos of 20 mm. the tubules of the dorsal and ventral pancreatic an- 

 lages interlock (Fig. 187 B). Eventually, anastomosis takes place between the 

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

 dorsal duct persist as the functional pancreatic duct of the adult. The proximal 

 portion of the dorsal pancreatic duct forms the accessory duct, which remains 

 pervious, but becomes a tributary of the adult 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 com- 

 prises the whole of the body and tail. 



The ventral pancreas may arise directly from the intestinal wall (Bremer; Keibel and 

 Elze), and paired ventral anlages also occur (Debeyre; Helly; KoUmann). Accessory pan- 

 creases 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. 



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 cellular 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 sur- 

 face is attached the transverse mesocolon. 



BODY CAVITIES, DIAPHRAGM AND MESENTERIES 

 The Primitive Coelom and Mesenteries. — In the Peters embryo the primary 

 mesoderm has already spht to form the extra-embryonic coelom (Fig. 74 C). 

 When the intra-embryonic mesoderm differentiates, numerous 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. Similarly, right and left pleuro-peritoneal cavities 

 are formed between the mesoderm layers caudal to the heart. The paired peri- 

 cardial cavities extend toward the midline cranial to the heart and communicate 

 with each other (Fig. 188). Laterally they are not continuous with the extra- 

 embryonic coelom, for the head of the embryo separates early from the underlying 

 blastoderm. The pericardial cavities also are prolonged caudally until they open 

 into the pleuro-peritoneal cavities. These in turn communicate laterally with 

 the extra-embryonic coelom. In an embryo of 2 mm. the ccelom thus consists of 

 a U-shaped pericardial cavity, the right and left limbs of which are continued 



