166 EDWARD MCCRADY, JR. 



veins in the body wall which empty into the external iliacs, 

 and two in the bladder which empty into the internal iliacs or 

 hypogastrics. McClure ('06) saw the two abdominal veins 

 in the body wall of an 8-mm. opossum (my stage 33). These 

 are strictly embryonic, disappearing .in stage 35. The two 

 which with their tributaries drain the wall of the bladder into 

 the hypogastric become the vesical veins of adult anatomy. 



The developmental history of this abdominal-umbilical 

 series of vessels has an interesting phylogenetic parallel. In 

 some elasmobranchs there is on each side of the body a lateral 

 abdominal (or epigastric) vein which takes blood from both 

 the pelvic and pectoral fins to the duct of Cuvier. This corre- 

 sponds to the umbilical vein in the opossum embryo of stage 

 28. In amphibians and reptiles the hindlimb drains into both 

 the abdominal and postcardinal veins, which is the situation 

 in the opossum of stages 28, 29 and 30. In Amphibia and 

 some reptiles the lateral abdominal veins may migrate to the 

 midventral line and fuse into a single anterior abdominal 

 vein which runs from the external iliac up to the liver and 

 through the latter to the sinus venosus (Kingsley, '26, p. 326). 

 In the opossum the umbilical veins thus unite in stage 34. 

 In the birds the same thing occurs, but in addition, the bladder, 

 instead of remaining entirely within the body in embryonic 

 stages, expands through the umbilicus to form an allantois 

 which, being in the path of this abdominal vein, utilizes it 

 and gives it a new significance. This as already described, is 

 precisely the history of the umbilical vein of the opossum 

 during the last 2 days of development. The paired, lateral, 

 abdominal veins thus become the single, ventral, umbilical. 



In the placental mammals this umbilical vein is the vein of 

 the placenta, which is formed from allantoic mesoderm. We 

 can, therefore, trace the placental vein of the highest verte- 

 brates back to the lateral abdominal vein of the sharks. The 

 location and course of this lateral abdominal vein, its connec- 

 tion with both limb buds in the opossum embryo, and with both 

 fins in the shark, suggest that if the paired limbs and paired 

 fins of vertebrates were evolved from a primitive paired fin- 

 fold, as is usually taught, then this vein was originally the 



