1887.] JEarly Development of the Pericardium, Sfc. 275 



of the placenta, which is developed in connexion with somatic struc- 

 tures. 



The further development of the ducts of Cuvier is then explained, 

 and those vessels are shown to end, as did the jugulars from which 

 thej are formed, by opening into the mouths of the umbilical veins 

 quite close to the heart. In the next stage of development, owing to 

 the expansion of the heart, the omphalomesenteric veins, umbilical 

 veins, and Cuvierian ducts, acquire separate openings into the heart, 

 and at the same time the right and left umbilical veins, just before 

 entering the heart, communicate with the venous spaces of the liver, 

 and have through them an alternative route to the heart. Whilst 

 these changes are in progress, the left omphalomesenteric vein, where 

 it is related to the liver, becomes occluded with liver substance. The 

 gradual conversion of the mesocardium laterale and septum traversum 

 into a dorsal pericardium and ventral 'diaphragm is then described, 

 and afterwards the closure of the iter venosum by the apposition of 

 the Cuvierian ducts and the sides of the trachea and oesophagus ; 

 whilst this is in progress the subclavian veins appear and empty 

 themselves into the Cuvierian ducts, which in this way become the 

 right and left superior venae cavae. 



During the twelfth day the umbilical veins lose their direct 

 opening iuto the heart, and the left vein, taking advantage of the 

 alternative route through the liver, passes through the substance of 

 that organ to end in the right omphalomesenteric vein close to the 

 heait. The channel which unites the left umbilical vein to the right 

 omphalomesenteric vein is the ductus venosus Arantii. When the 

 permanent kidneys and hind limbs develop, a vein passes from them 

 into the cardiac end of the right omphalomesenteric vein, so that it 

 becomes the terminal end of the inferior vena cava. 



Whilst these changes are in progress numerous mesenteric veins 

 develop, and open into the hinder portion of the right omphalomes- 

 enteric, which then becomes the portal vein, and at first empties into 

 the sinus venosus Arantii. The hepatic portion of the left omphalo- 

 mesenteric vein is quite obliterated, and that vessel ceases to enter 

 the heart ; however, its hinder part may persist and carry blood from 

 the mesentery into the portal vein, with which it acquires com- 

 munications. 



About the middle of the twelfth day, and when the iter venosum is 

 upon the point of closure, the dorsal diaphragm develops as a crescentic 

 fold projecting from the side body- wall close to the superior vena3 

 cavae, and uniting the dorsal pericardium to the dorsal body- wall. 

 As the thorax develops this dorsal diaphragm travels further tail- 

 wards, its hindermost dorsal attachments being united to the fore- 

 end of the urogenital ridge, and its ventral attachments with the 

 dorsal part of the liver and the mesoblast which covers it. 



