72 



EMBRYOLOGY 





border of the fore-limb is directed lateralward, and the preaxial (tibial) border 

 of the hind-limb is directed medialward; thus the flexor surface of the fore-lima 

 is turned forward, and that of the hind-limb backward. 



DEVELOPMENT OF THE BODY CAVITIES. 





In the human embryo described by Peters the mesoderm outside the embryonic 

 disk is split into two layers enclosing an extra-embryonic coelom; there is no trace 

 of an intra-embryonic coelom. At a later stage four cavities are formed within the 

 embryo, viz., one on either side within the mesoderm of the pericardial area, and 

 one in either lateral mass of the general mesoderm. All these are at first independent 

 of each other and of the extra-embryonic celom, but later they become continuous. 

 The two cavities in the general mesoderm unite on the ventral aspect of the gut 

 and form the pleuro-peritoneal cavity, which becomes continuous with the remains 

 of the extra-embryonic celom around the umbilicus; the two cavities in the peri- 

 cardial area rapidly join to form a single pericardial cavity, and this from two lateral 

 diverticula extend caudal ward to open into the pleuro-peritoneal cavity (Fig. 54). 



Mesentery 



Mesoderm 



surrounding 



duct ofCuvier 



"""- Dorsal mesocardium 



Pleuro- pericardial 

 opening 



Pericardium 



FIG. 54. Figure obtained by combining several successive sections of a human embryo of about the fourth week 

 (From Kollmann.) The upper arrow is in the pleuroperitoneal opening, the lower in the pleuropericardial. 



Between the two latter diverticula is a mass of mesoderm containing the ducts 

 of Cuvier, and this is continuous ventrally with the mesoderm in which the umbili- 

 cal veins are passing to the sinus venosus. A septum of mesoderm thus extends 

 across the body of the embryo. It is attached in front to the body-wall between 

 the pericardium and umbilicus; behind to the body-wall at the level of the second 

 cervical segment; laterally it is deficient where the pericardial and pleuro-peri- 

 toneal cavities communicate, while it is perforated in the middle line by the fore- 

 gut. This partition is termed the septum transversum, and is at first a bulky plate 

 of tissue. As development proceeds the dorsal end of the septum is carried grad- 

 ually caudalward, and when it reaches the fifth cervical segment muscular tissue 

 with the phrenic nerve grows into it. It continues to recede, however, until it 

 reaches the position of the adult diaphragm on the bodies of the upper lumbar 

 vertebra?. The liver buds grow into the septum transversum and undergo 

 development there. 



The lung buds meantime have grown out from the fore-gut, and project laterally 

 into the forepart of the pleuro-peritoneal cavity; the developing stomach and liver 

 are imbedded in the septum transversum; caudal to this the intestines project into 

 the back part of the pleuro-peritoneal cavity (Fig. 55) . Owing to the descent of 



