160 



From the beginning the right and left supracardinals differ 

 in a few minor respects. The right vessel gains access to the 

 common cardinal through a short segment of the postcardinal, 

 which segment is thus preserved and incorporated into the 

 right azygos vein. The left supracardinal, on the other hand, 

 drains directly into the common cardinal, and the entire 

 thoracic portion of the postcardinal degenerates. Both supra- 

 cardinals are sometimes preserved in the adult opossum, but 

 the left is always the better developed of the two. 



The paravertebral lymphatics and the thoracic duct. In 

 connection with the dropping out of the lateral radices of the 

 original intersegmental tributaries to the postcardinal, 

 Zimmermann has pointed out that the degenerating vessels go 

 through a history quite like that of the paracardinal plexus. 

 That is to say, they become detached from the stem veins, 

 break up into sacculated, discontinuous, remnants surrounded 

 by the usual peri-intimal spaces. These spaces in the usual 

 way develop an endothelial lining, sprout longitudinally, and 

 coalesce to form a lymphatic channel running longitudinally 

 on the lateral side of the sympathetic trunk. This new channel 

 he calls the paravertebral lymphatic. 



At the time of birth (stage 35, 17173) the left paravertebral 

 lymphatic has grown craniad around the dorsal side of the 

 left azygos vein and attained communication with the left 

 thoracic duct. The right at this time is still independent. 

 Both vessels receive the intercostal lymphatics which are ap- 

 parently derived as sprouts from them. 



When in stage 33 the supracardinals replace the post- 

 cardinals in the thoracic region, ventromedial sprouts from 

 them into the mediastinum serve, as in the case of the similar 

 ones from the postcardinals, as the skeleton around which 

 more anlagen of the thoracic duct form. The thoracic duct 

 thus grows craniad. Its most cranial anlagen in stage 33 are 

 located at the level of the junction of the common cardinal 

 with the sinus venosus, and instead of being grouped together 

 in the midsagittal plane, they tend to separate into two 

 bilaterally symmetrical channels. This foreshadows the fact 

 that in the adult opossum the thoracic duct branches sym- 

 metrically at its anterior end to empty into both jugular lymph 

 sacs. 



Migration of the gill pouch derivatives. In early stage 32 

 (16174) the third gill pouches become detached from the 

 pharynx (fig. 47, G). In late stage 32 and in stage 33 the 



