1 88 THE ENTODERMAL CANAL AND ITS DERIVATIVES 



the dorsal surface of the tail. The alveoli of the gland are developed as darkly staining cellular 

 buds in embryos of 40 to 55 mm. The islands characteristic of the pancreas appear first in the 

 tail at 55 mm. 



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. 



Pericardial Cai/ifu 



Surface of 

 fore-gut 



Pleuro- pericardia t 

 Canal 



Entoderm of gut 



Peritonea I Cavity 



1 



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. 232). 

 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 cavi- 

 ties 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 ex- 

 tend toward the midline cranially and com- 

 municate with each other (Fig. 181). They 

 also are prolonged caudally until they open 

 into the pleuro-peritoneal cavities. These in 

 turn communicate laterally with the extra- 

 embryonic ccelom. In an embryo of 1.5 mm. 

 the ccelom thus consists of a U-shaped peri- 

 cardial cavity, the right and left limbs of which 

 are continued caudally into the paired pleuro-peritoneal cavities ; these extend 

 out into the extraembryonic ccelom. 



When the head-fold and fore-gut of the embryo are developed, the layers of 

 splanchnic mesoderm containing the heart tubes are folded together ventral to 

 the fore-gut and form the ventral mesentery between the gut and the ventral body 

 wall (Fig. 182). Owing to the position of the yolk-sac, the caudal extent of 

 the ventral mesentery is limited. At the level on each side, where the vitello- 

 umbilical trunk courses to the heart, the splanchnic mesoderm and the somatic 

 mesoderm are united (Fig. 182). Thus is formed the septum transversum, which 

 separates the ventral mesentery into a cranial and caudal portion. Cranial to 

 the septum, the heart is suspended in the ventral mesentery which forms the 

 dorsal and ventral mesocardia (Fig. 183 A). Into the ventral mesentery caudal 



xtra-embryonic 

 Coelam 



-Wall of yol K-sac 



Fig. 181. — Diagrammatic dor- 

 sal view of the ccelom in an early 

 human embryo (modified alter Rob- 

 inson). 



