Mesenteric Sac and Thoracic Duct in Embrvo Pig 



311 



greater fusion with tlie formation of several definite channels bearing a 

 close resemblance to the sac in the later stages; but still having large 

 openings into the veins. This is shown in Figs. 4 and 5; in Fig. 4, 

 representing a section through the renal anastomosis of the sub-cardinal 

 veins, in an embryo of 20 mm., the root of the mesentery is occupied by 



Fig. 4. — Transverse section through the renal anastomosis of the sub-car- 

 dinal veins in an embryo pig, 20 mm. long. In this section the venous chan- 

 nels in the root of the mesentery are beginning to show definite evidences 

 of fusion and sac formation, though they are still connected with the veins, 

 as is shown in the figure. A., aorta ; R. A., renal anastomosis ; W. B., Wolffian 

 body ; M. C, mesenteric capillaries ; Mes., mesentery ; O. A., genital anlage. 



large venous channels between which there are extensive communications. 

 At this stage there are still numerous openings into the veins from which 

 the vessels can all be definitely injected, but not as easily as in the earlier 

 embryos, since the venous connections are now being gradually oblit- 

 erated. One of these openings is shown in the figure, with the ink 

 entering the sac. In Fig. 5, taken from a section a little lower down in 



