FROM TWELVE TO THIRTY-SIX SOMITES 207 



the entire length of the alimentary canal, while the ventral 

 mesentery persists only in the region of the fore-gut and the 

 cloaca. 



The embryonic body-cavity shows two divisions from a very 

 early stage, viz., (1) the large cephalic or parietal cavity situated 

 in the pharyngeal region of the head and containing the heart, 

 and (2) the general coelomic cavity of the trunk. After 

 the heart is established in the middle line the parietal cavity 

 is bounded posteriorly by the wall of the anterior intestinal portal 

 (Figs. 75, 85, etc.), but it communicates with the pleuro peri- 

 toneal cavity around the sides of the portal, in which the vitelline 

 veins run. Laterally the parietal cavity communicates with the 

 extra-embryonic body-cavity. 



The mesocardia lateralia are also an important landmark in 

 the embryonic body-cavity because from them proceed the par- 

 titions that subsequently separate the pericardial and pleura! 

 cavities on the one hand, and the pleura! and peritoneal body- 

 cavities on the other. (See Chap. XL) The primordium of the 

 lateral mesocardia may be recognized in the 10 s stage : just behind 

 the heart the median portion of the body-cavity is thick-walled, 

 the peritoneal cells being actually columnar. At this place, a 

 short distance lateral to the median angle of the body-cavity, 

 and at the junction of the cylindrical and flat mesothelium, a 

 fusion of considerable longitudinal extent is formed between 

 the somatopleure and the proximal portion of the vitelline veins, 

 projecting up from the splanchnopleure; this fusion is the begin- 

 ning of the lateral mesocardium. It separates a more median 

 portion of the body-cavity from a more lateral, and in it the 

 duct of Cuvier soon develops. 



When this portion of the body of the embryo becomes ele- 

 vated (forty to fifty hours) the portion of the body-cavity lateral 

 to the mesocardia lateralia comes to lie ventrally to the median 

 portion (cf. Fig. 69), and at the same time the lateral mesocardia 

 rotate around a longitudinal axis through an angle of about 

 90, so that the original median border becomes dorsal, and the 

 original lateral border becomes ventral. The dorsal divisions, 

 right and left, of the pleuro peritoneal cavity may now be called 

 the pleural grooves. Inasmuch as the parietal cavity has receded 

 considerably at the same time into the trunk with the elongation 

 of the fore-gut, it comes to lie beneath the pleural grooves 



