THE EMBRYOLOGY OF THE OPOSSUM 135 



The chondrostyle is forming around the notochord, and pre- 

 muscle masses are prominent in the forelimb (fig. 42, A, B, 

 C and D). 



Figure 42, J, is a reconstruction of the wolman duct and the 

 blood vessels of the lumbosacral region. The wolman duct is 

 the very slender, black tube. The aorta and its branches are 

 shaded with curved lines. The umbilical vein in stippled. The 

 postcardinal vein is unshaded. It can be seen that the aorta, 

 which became single in the lumbar region in stage 29, is in 

 stage 30 still double at the level of origin of the common iliac 

 arteries. The paired aortae are continued into the tail as 

 the caudal arteries which run on either side of the tail gut. 

 The common iliac arteries divide at the level of the limb bud 

 into the external iliac and the hypogastric. The latter passes 

 ventrally and cranially around the cloaca and out to the al- 

 lantois. The single, median, caudal vein runs on the ventral 

 side of the tail gut (section G), divides near the cloaca 

 (section H) into the two postcardinals, which latter receive 

 many small veins from the limb bud before they reach the 

 mesonephros. 



The capillary connections between the omphalomesenteric 

 veins dorsal to the gut anterior to the liver bud, and ventral 

 to the gut posterior to the liver bud, described in stage 29 

 (p. 123) have now developed into large anastomoses, so that 

 the veins form two complete loops around the duodenum. 

 However, the left arm of the anterior loop has already become 

 plexiform, as described in connection with the liver cords. 



A slight thickeing of the peritoneum in a longitudinal line 

 along the medial wall of the mesonephros is the anlage of the 

 genital ridge. 



The tail gut is at the height of its development (central re- 

 construction). 



The second half of the eleventh day 



Stage 31. External distinctive features. Changes in the 

 chorion. Changes in the venous system. The earliest 

 lymphatic anlagen. Changes in the arterial system. Changes 



