THE EMBRYOLOGY OF THE OPOSSUM 167 



vein of the finfold. At any rate, it is the original vein of the 

 limbs from whatever source they came. 



The diaphragm. The origin of the septum transversum by 

 the lateral and ventral expansion of the sinus venosus, the 

 origin of the pleuropericardial membrane by the medial 

 migration of the ducts of Cuvier ; and of the pleuroperitoneal 

 membrane as a ridge which forms at the junction of the 

 pleuropericardial membrane and the lateral body wall, have 

 already been described. 



The pleuriperitoneal membrane was described as continuous 

 anteriorly with the anterior tip of the mesonephros, and pos- 

 teriorly with the coronary appendage of the liver (i.e., dorsal 

 pillar of the diaphragm). Between these two ends the edge 

 of the membrane was in stages 31 to 33 directed toward the 

 interval between the lungs and the mesonephros. Between 

 stages 33 and 34 a considerable elongation of the trunk occurs, 

 which results in carrying the mesonephros caudad away from 

 the lungs. This movement draws the pleuroperitoneal mem- 

 brane caudad around the posterior tip of the lung. It then 

 fuses with the dorsal mesentery and separates the pleural 

 cavity completely from the peritoneal cavity. Muscle tissue 

 appears in the pleuroperitoneal membranes at this time, and 

 the diaphragm is complete. 



The perineum. Although figure 54 is taken from a speci- 

 men of stage 35, except for the prominence of the phallus it 

 will equally well represent stage 34. Here it may be seen 

 that the tail gut has completely disappeared and the perineal 

 fold has advanced ventrally until it has divided the cloaca into 

 a cranial urinogenital sinus and a caudal rectum. It is im- 

 portant to notice that the true cloaca has thus been eliminated 

 before birth and the anal and urinogenital apertures are 

 separate. There is a widespread impression, which dates at 

 least from the time of Selenka, that the adult female opossum 

 has a cloaca. This is due to a misleading appearance caused 

 by a common sphincter muscle which develops secondarily 

 around the anal and urinogenital apertures as will be de- 

 scribed in the chapter on the pouch young. 



