Mesenteric Sac and Thoracic Duct in Embryo Piff 



313 



appearance to that seen in Fig. 7, though this is taken from an older 

 embryo. At the 21 mm. stage, however, there are still definite venous 

 openings, although they are gradually becoming lessened by a gradual 

 process of differentiation which becomes complete between 22 and 23 

 mm., before the appearance of any uninjected vessels in the region dorsal 

 to the aorta, thus leaving the sac, for a short time, independent of either 

 venous or lymphatic connections (Figs. 7 and 8). Fig. 7 shows this 

 sac, with its irregular margins, located in the root of the mesentery just 





m 





^21, 







-^jy 



Fig. 6. — Transverse section through the mesonephric arteries in an embryo 

 pig, 22 mm. long. The mesenteric channels at this stage were uninjected, as 

 shown in the figure, while the adjoining veins were filled with blood or 

 injection mass. The venous connections of the sac have been entirely oblit- 

 erated. A., aorta ; W. B., Wolffian body ; S. C. V., sub-cardinal veins ; M. 8., 

 mesenteric sac ; Mes., mesentery ; M. A., mesonephric arteries ; P. G. V., post- 

 cardinal veins ; G. A., genital anlage. 



ventral to the sub-cardinal veins, that is, in the exact location of the 

 venous plexus in all the earlier stages. It has lost all venous connections 

 and has not yet acquired any communication with the rest of the lym- 

 phatic system, but exists for a short time, as an independent sac in the 

 embryonic mesenchyma. Fig. 8 is a section from a lower level in the 

 same embryo and shows the beginnings of the mesonephric arteries with 

 the area between occupied by an independent sac which has replaced the 

 large venous channels of the preceding stages. 



