PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 341 



body cavity). From the nature of the bending process, the embryonic coelom 

 is divided into bilaterally symmetrical parts by the common mesentery (Fig. 

 197) . These two simple cavities are the forerunners of all the serous cavities of 

 the body. The various partitions between the serous cavities, the walls of the 

 cavities and the mesenteries proper are all derived from the somatic and 

 splanchnic mesoderm with its covering of mesothelium. 



While the foregoing would represent a typical case of early ccelom and 

 mesentery formation, there are certain modifications and peculiarities in the 

 higher Mammals and in man. In all cases the splitting of the mesoderm to 

 form the coelom proceeds from the periphery of the germ disk toward the axial 

 portion (p. 80) . In the human embryo the bending ventrally and fusing of the 

 germ layers to form the cylindrical body begins in the anterior region of the 

 disk and is accomplished there before the splitting of the mesoderm is com- 

 pleted. The peripheral splitting has resulted in the formation of the exoccelom, 

 but at the time when the ventral fusion of the germ layers takes place, the split- 

 ting has not extended axially to a sufficient degree to form the intraembryonic 

 crelom. The latter, which appears later in this region, never communicates 

 laterally, therefore, with the exocoelom. Caudal to this region the coelom is 

 formed as in the typical case. The more anterior part of the coelom on each 

 side is thus primarily a pocket-like cavity. It communicates with the rest of the 

 coelom at about the level of the yolk stalk. In the region of the fore-gut, the 

 future oesophagus, no distinct mesentery is formed, but the fore-gut remains 

 broadly attached to the dorsal body wall. A ventral mesentery is lacking from 

 a point just cranial to the yolk stalk to the caudal end of the gut. There are 

 no coelomic cavities in the branchial arches, the ccelom extending only to the 

 last branchial groove. 



In very young human embryos the primitive segments contain small cavities. 

 These cavities soon disappear, being filled with cells from the surrounding, 

 parts of the segments. Whether they represent isolated portions of the coelom 

 is not certain. In the lower Vertebrates, the cavities of the primitive segments 

 regularly communicate with the ccelom, and in the sheep the cavities of the first 

 formed segments are continuous with the ccelom. In the head there is no 

 cavity analogous to the coelom in the body. In but one human embryo have 

 any cavities in the head resembling those of the primitive segments been 

 observed (see p. 2 70) . 



The Pericardial Cavity, Pleural Cavities and Diaphragm. 



The pericardial and pleural cavities and diaphragm are so closely related in 

 their development that they must be considered together. In the region just 

 caudal to the visceral arches, where the two anlagen of the heart appear, the 

 embryonic coelom becomes dilated at a very early stage to form the primitive 

 pericardial cavity (parietal cavity of His). After the two anlagen of the heart 



