140 EDWAED MCCRADY, JR. 



adult anatomy, which may be seen in that position in the re- 

 construction in figure 45. But I have not a sufficiently closely 

 graded series between stages 30 and 31 to establish this point, 

 and as far as the evidence goes it seems more likely that 

 the cephalic is a new vein. 



The primitive ulnar vein has migrated along the post- 

 cardinal to the promontory. And the inferior jugular has 

 migrated up the duct of Cuvier until it has become a tributary 

 to the precardinal (fig. 45, reconstruction). 



The earliest lymphatic anlagen. The origin, development, 

 and adult anatomy of the lymphatic system in the opossum 

 has been worked out in great detail by Dr. A. A. Zimmermann 

 of the University of Illinois Medical College. This work was 

 done partly at The Wistar Institute and partly at his own 

 University. He very generously allowed me to read his manu- 

 script before it went to press, and to incorporate an abstract 

 of his embryological observations and conclusions in this book. 

 The account of the development of the lymphatics in this and 

 succeeding chapters is based entirely upon his work, and I am 

 very much indebted to him for the privilege of citing it here. 



The first steps in the development of the jugular lymph sacs 

 and of the thoracic duct occur independently of each other 

 during stage 31. In stages 27 and 28 there was formed at the 

 angle between the pre- and post-cardinals a plexus of veins 

 which I called the paracardinal plexus (p. 110). In stage 31 

 this plexus is divided into dorsomedial and dorsolateral por- 

 tions. The former is the more cranial of the two. It is lateral 

 to the intersegmental veins 1 to 5, and medial to the cervical 

 nerves 4 and 5. Anteriorly it communicates with the first 

 intersegmental vein and posteriorly with the jugular prom- 

 ontory near the mouth of the external jugnilar (jugulo- 

 caphalic) vein. The dorsolateral portion is medial to the 

 blastema of the scapula and lateral to the cervical nerves 3 

 to 7. It communicates with the jugular promontory dorso- 

 lateral to the common stem of the third and fourth inter- 

 segmental veins (fig. 45). This connection coincides with the 

 mouth of the primitive ulnar vein. It also communicates by 

 a small venule with the dorsomedial portion described above. 



