536 



In describing the stages which just precede and which lead up 

 directly to a definite establishment of the thoracic and right lymphatic 

 ducts, we will begin with a 14 mm embryo. In the 14 mm embryo, 

 as intimated above, there is as yet no indication of a definitely 

 established thoracic or right lymphatic duct, either in front of or 

 behind the cardinal-Cuvierian junction. In their place, however, there 

 are present a number of venous outgrowths which are in the process 

 of being split off from the main venous channels, exactly in the line 

 followed by these ducts in the 16 mm embryo. These venous out- 

 growths, in the writer's estimation, constitute the veno-lymphatic an- 

 lages of the thoracic and right lymphatic ducts. The reconstruction 

 of the 14 mm embryo (Fig. 2) shows the presence of the venous out- 

 growths which arise along the dorsal 

 surface of the left common jugular and 

 innominate veins and, when compared 

 with the reconstruction of the 16 mm 

 embryo, it is seen that they occupy in 

 this region the same relative position 

 with respect to the main venous channel, 

 the A. thyro-cervicalis and the sympathetic 

 nerve, as does the thoracic duct in the 

 16 mm embryo (Fig. 1). These relations 

 are further borne out by a study of the 

 following microphotographs of transverse 

 sections of the 14 and 16 mm embryos 

 taken, approximately, at corresponding 

 levels, as indicated by arrows in Figs. 1 

 and 2. 



Figs. 3 and 4, of a 14 and 16 mm 

 cat embryo i), respectively, represent trans- 

 verse sections through the body in the 

 neighborhood of the left jugular lymph 

 sac where the outgrowth from the left common jugular vein (veno- 

 lymphatic anläge of thoracic duct) and the thoracic duct both lie be- 

 tween the A. thyro-cervicalis and the sympathetic nerve. Figs. 5 and 

 6, also of a 14 and 16 mm cat embryo, respectively, represent trans- 



Fig. 2. Dorsal view of a 

 reconstruction of a 14 mm cat 

 embryo, made after the method 

 of Born, showing the posterior 

 portion of the left jugular lymph 

 sac and the main venous channel 

 contiguous to it which, in part, 

 constitutes the future internal 

 jugular, common jugular and in- 

 nominate veins of the left side. 



1) The sections represented by Figs. 3, 4, 5 and 6 were photo- 

 graphed from the anterior surface so that the medial side of the left 

 common jugular and innominate veins is directed toward the left side 

 of the figure. If this circumstance is borne in mind no confusion will 

 result when comparing these figures with Figs. 1 and 2. 



