GENERAL EMBRYOLOGY 1643 



that the bloodvessels concerned in it are the two umbilical arteries 

 and the two timbilical veins (there being two such veins at an early 

 period) . 



The umbilical arteries replace the allantoic arteries, and they 

 originally grow from the caudal ends of the two primitive dorsal 

 aorta. They carry impure blood from the foetus to the placenta, 

 where it is purified and furnished with nutritive materials from the 

 maternal blood b\- a process of osmosis. 



The umbilical veins replace the allantoic veins, and they convey 

 pure blood from the placenta to the body of the foetus. At an early 

 stage, to which this description only applies, they terminate in the 

 sinus venosus. These umbilical veins have been already referred 

 to in connection with the development of the veins. 



Development of the Pericardial, Pleural, and Peritoneal Cavities, 

 Septum Transversum, and Diaphragm. 



The coelom, body-cavity, or pleuro-peritoneal cavity is the space 

 which lies between the somatopletire of the body-wall and the 

 splanchnopleure of the primitive alimentary tube. It is originally 

 a single ca\'ity, but is subsequently divided into (i) the pericardial 

 cavity ; {2) the pleural cavities, right and left ; and (3) the peritoneal 

 ca\4ty. The pericardial and pleural cavities are situated in the 

 thorax, and the peritoneal cavity is situated in the abdomen, the 

 septum between the thorax and the abdomen being formed by the 

 diaphragm. 



Pericardial Cavity. — In connection with the development of the 

 heart, it has been stated that the rudiments of that organ take the 

 form of two tubes, which are hollowed out in the splanchnic meso- 

 derm. These two tubes, approaching each other, fuse and form a 

 single longitudinal tube, or primitiv'e heart, which lies mesially along 

 the ventral aspect of the phar>Tigeal part of the fore-gut. It has 

 been further stated that the primitive heart is connected with the. 

 ventral body-wall b}' a bilaminar fold of splanchnic mesoderm, called 

 the ventral meoscardium, and with the phar^Tigeal part of the fore- 

 gut by another similar fold, called the dorsal mesocardium. These 

 folds soon disappear — the ventral mesocardium entirely, and the 

 dorsal mesocardium to a large extent. There are also two lateral 

 mesocardia — right and left — which are continuous with the ventral 

 margin of the septum transversum, each of these containing a duct 

 of Cuvier on its way to the sinus venosus of the heart. 



The heart is developed in the splanchnic mesoderm of the peri- 

 cardial region (cardiac portion of the ventral mesentery), the portion 

 of that mesoderm which is ventral to the primitive heart being the 

 ventral mesocardium, whilst the portion dorsal to the primitive 

 heart forms the dorsal mesocardium. The part of the coelom which 

 corresponds to the pericardial region is known as the pericardio- 

 thoracic cavity, and it gives rise to the pericardial cavity. This latter 

 cavity is subsequently shut off from the peritoneal cavity by the 



