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-cut 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 ccelomic 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, ete.), but it communicates with the pleuroperi- 
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 bedy-cavity because from them proceed the par- 
titions that subsequently separate the pericardial and pleural 
eavities on the one hand, and the pleural and peritoneal body- 
cavities on the other. (See Chap. XI.) The primordium of the 
lateral mesocardia may be recognized in the 10s 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- 
rated (forty to fifty hours) the portion of the body-cavity lateral 
to the mesocardia lateralia comes to le 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 pleuroperitoneal 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 
