72 



HUMAN EMBKYOLOGY. 



when the umbilical orifice closes. The extra-embryonic portion is entirely 

 obliterated when the outer surface of the expanding amnion fuses with the inner 

 surface of the chorion (compare Figs. 77 and 78). 



The Intra- embryonic Ccelom. The intra-embryonic coelom appears as a series 

 of cleft-like spaces in the margin of the embryonic mesoderm. The spaces fuse 

 together to form a fl -shaped cavity (Fig. 89) which separates the peripheral part 

 of the embryonic mesoderm into a parietal or somatic, and a visceral or splanchnic, 



layer. The bend of the D -shaped cavity lies in the 

 margin of the cephalic part of the embryonic region, 

 and it has no direct communication with the extra- 

 embryonic coelom, but the greater part of each stem 

 of the cavity, on account of the disappearance of its 

 lateral wall, soon opens, laterally, into the extra- 



-peritoneal canal em bryonic CCelom. 



The transverse portion of the n -shaped cavity, which 

 extends across the cephalic end of the embryonic area 

 and connects the two limbs together, is the pericardial 

 cavity. The adjacent part of each lateral limb of the 

 cavity is the pleuro-pericardial canal, it becomes a pleural 

 cavity, and the remaining portions of the two limbs 

 SCHEMA OF INTRA- unite ventrally, as the umbilical orifice closes, to form 



EMBRYONIC C(ELOM SEEN FROM r v, p ei ny u npritnnpal pa-n-i-Hr 



ABOVE BEFORE THE FOLDING OF l "H* Pineal Cavity. 



THE EMBRYONIC AREA. As the head fold forms, the pericardial part of the 



cavity is carried ventrally and caudally into the ventral 



wall of the fore-gut (Fig. 90). The mesoderm which originally formed its peri- 

 pheral boundary, but which now lies in the cephalic boundary of the umbilical 

 orifice, becomes thickened, and forms the septum transversum (Figs. 90, 91, 93). 



Alimentary 

 canal 



Alimentary 

 canal 



I 1 Pleuro- 



Peritoneal 

 coelom 



FIG. 89. 



Pericardium 

 Opening into pleuro- 

 peritoneal canal 

 Pleuro-peritoneal canal 



Peritoneum 



Spinal medulla 

 ore -gut 



_ Pleuro-pericardial 

 canal 



Spinal medulla 



Dorsal mesentery 

 Alimentary canal 

 Peritoneum 



pinal medulla 

 Fore-gut 



Pleuro- 

 pericardial canal 



Heart 



Pericardial 

 cavity 



FIG. 90. SCHEMATA OF EMBRYONIC CCELOM AFTER FOLDING OF EMBRYONIC AREA BUT BEFORE THE 

 SEPARATION OF THE VARIOUS PARTS. D from above ; A, B, and C at levels of line A, B, and C in Fig. D. 



At the cephalic end of its dorsal wall, on each side, the pericardial cavity is still 

 continuous with the two lateral parts of the coelom ; and each lateral part, which 



