THE EMBRYOLOGY OF THE OPOSSUM 87 



resulted in the fusion of the heart tubes in their bulbar por- 

 tions, and the formation of a pharyngeal floor as far caudad 

 as the second branchial pouch. During stage 26 this same 

 process carries the fusion of the hearts through the ven- 

 tricular region, and the floor of the pharynx beneath the pos- 

 terior branchial complex a dorsolateral outpocketing of the 

 pharynx from which the third, fourth and fifth branchial 

 pouches will be derived (fig. 28, reconstruction). At the same 

 time the heart twists slightly so that the bulb is pushed to 

 the right and the ventricle to the left. The atria and sinuses 

 of the hearts are still separate, and the floorless portion of 

 the pharynx immediately between them is the portion in which 

 the lung pouches are forming. 



Origin of the postcardinal vein. The general manner of 

 origin of the postcardinal has already been indicated in a 

 previous section. It is part of a primary longitudinal venous 

 complex which originates beneath the floor of the mesen- 

 cephalon and grows caudad on the medial side of the sensory 

 ganglia and the otocyst. As it reaches the level of the anterior 

 end of the umbilical vein it forms capillary communications 

 with the latter and thereby gains access to the sinus venosus ; 

 but other capillary sprouts from it continue caudad to the an- 

 terior level of the limb ridge fourth somite. Here they 

 invade the dorsal part of the limb. This seems to be their 

 primary association in this part of the body, but as the 

 nephric anlagen are developing from the seventh somite 

 caudad, the postcardinal venules, still growing caudad, soon 

 become associated with them also. In later stages the post- 

 cardinal retains its connection with the dorsal part of the 

 forelimb and establishes a similar connection with the dorsal 

 part of the hindlimb, but the association with the developing 

 mesonephros becomes much the most important. 



There is still one group of branches from the postcardinal 

 plexus which have not been mentioned. At intersomitic levels 

 capillary sprouts run dorsomedially between the somites and 

 communicate with similar sprouts from the aortae. These 

 are the precursors of the segmental veins. 



