314 



Walter A. Baetjer 



Between 23 and 25 mm. the thoracic duct appears, extending at first 

 from about the level of the renal anastomosis up to a point approximately 

 opposite the arch of the aorta. It is bilateral in the thoracic region, 

 but fuses below the diaphragm to form a much dilated channel, just 

 dorsal to the aorta — probably the primitive receptaculum. From 35 

 mm. on the sac gradually develops its connection with the rest of the 



Fig. 7. — Transverse section through the renal anastomosis, in an embryo 

 pig, 23 mm. long. This is the first appearance of a definite sac in the exact 

 location of the venous plexus in the earlier stages. It will be noticed that 

 the irregular margins suggest the fusion of many small vessels. At this stage 

 no connection can be traced between the sac and either the lymphatic system 

 or the veins. A., aorta ; R. A., renal anastomosis ; M. f>., mesenteric sac ; 

 W. B., Wolffian body. 



lymphatic system in the following manner: small capillaries grow from 

 the sac and primitive receptaculum; these are shown in the earliest stage 

 in Fig. 8 as small vessels extending up from the sac, on each side of the 

 aorta, where they eventually meet and anastomose, finally forming defi- 

 nite channels along the lateral margins of the aorta, complete by 30 mm. 

 (Fig. 9) from which stage they can be definitely injected by way of the 

 thoracic duct. 



Prior to this time, however, when the embryo is between 25 and 26 

 mm. in length, communication is established between the early thoracic 



