540 



the azygos veins. Since these outgrowths possess a still wider range 

 of development in the thoracic region, in embryos measuring between 

 14 and 16 mm in length, and are then, almost instantaneously, suc- 

 ceeded in the 16 mm embryo by definite and continuous lymph channels, 

 it appears that there can be little doubt as to their being the anlages 

 of the thoracic and right lymphatic ducts. 



The slight projection or extension of the jugular lymph sac in the 

 14 mm embryo (oc, Fig. 2) ends blindly and is the point at which the 

 lymph sac subsequently connects with the independently formed thoracic 

 duct. This projection is met with in all embryos in which a jugular 

 lymph sac has been established. The youngest embryo in which it 

 was observed by the writer measured 10.7 mm in length (Harvard 

 Embryological Collection, Series 474). 



Before explaining the manner in which the veno-lymphatics are 

 transformed into the thoracic and right lymphatic ducts we will first 

 consider how they arise from the veins and the order in which they 

 make their appearance. It may be stated at the beginning that the 

 veno-lymphatic anlages of the thoracic and right lymphatic ducts first 

 make their appearance along the dorsal surface of the common jugular 

 and innominate veins. 



In the 12 mm embryo the veno-lymphatic anlages of the ducts 

 are in the form of a ridge which is confined exclusively to the dorsal 

 surface of the common jugular and innominate veins. This ridge pro- 

 jects from the vein along the line of implantation of the latter's dorsal 

 segmental tributaries. These tributaries, when compared with those 

 of earlier stages show marked signs of atrophy. Along its posterior 

 portion, this veno-lymphatic ridge is separated from the main venous 

 channel by a number of fenestrae. 



In the 13 mm embryo the veno-lymphatic ridge which lies along 

 the dorsal surface of the common jugular and innominate veins pre- 

 sents about the same appearance as in the 12 mm embryo. In ad- 

 dition to it, however, but not directly continuous with it, a ridge has 

 made its appearance along the ventral surface of each azygos vein at 

 about the middle of the thoracic region. This ridge is not continuous 

 along its entire extent on either azygos vein, but consists of a series 

 of independent outgrowths which are separated from the main venous 

 channel by a number of fenestrae. These independent outgrowths lie 

 exactly in the line subsequently followed by the thoracic and right 

 lymphatic ducts and constitute their veno-lymphatic anlages. 



In the 14 mm embryo the veno-lymphatic ridge which lies along 

 the common jugular and innominate veins in the 12 and 13 mm em- 



