244 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



below the pharynx and ends behind on the hind gut (Pig. 201). 

 This cavity is originally divided into a right and left half by 

 the dorsal and ventral mesentery, formed by the united 

 splanchnopleures (Pig. 192). The ventral mesentery disappears 

 in the anterior and posterior parts, so that the right and left 

 halves of the coelom communicate ; an intermediate part persists 

 and helps to form the diaphragm (Fig. 202). 



The Divisions of the Coelom. — Out of the anterior part of 

 the coelom (Fig. 201) is formed the pericardium. It lies beneath 

 and behind the primitive pharynx (Fig. 202). The pericardial 

 part of the coelom by the third week (Fig. 201) is expanded and 

 thrust downwards and backwards, and communicates by a narrow 

 neck or isthmus (iter venosum) with the pleuro-peritoneal part on 

 each side of the mesentery. The duct of Cuvier encircles the neck 

 or isthmus on each side, and causes the constriction (Pig. 183). 



1st aortic arch 



dorsal aorta 



I 



transverse sinus 



,left duct of Cuvier 

 <jPost. msso-/ 

 cardium / j ung bud in mesentery 



dorsal mesogast. 



rtom. 



art meso-card. 



con. arter. 



pencard. s j nus uenosus 

 amnion 



vitelline vein 

 liver bud 

 'yolk sac. 



ventral mesentery 



septum transv. 



Fl0 'p„°vi;TStf'T *?>, Sh H *h m ^f r ln which the Heart is fi *«i within the 

 pericardium by the Arterial and Venous Mesocardia in a human embryo of 3 



This condition is permanent in many fishes. With the origin of 

 the lungs the part of the coelom which forms the isthmus undergoes 

 a great expansion by the ingrowth of the lung buds (Fig. 201). 

 Thus out of the isthmus of the right and left sides are gradually 



