Mesenteric Sac and Thoracic Duct in Embryo Pig 



309 



more pronounced stage in this development is shown in Fig. 3 from an 

 embryo of 19 mm. During this time, also, there is a beginning of the 

 process of fusion — a process which eventually leads to the formation of 

 the sac in the later stages — by which many of the smaller vessels first 

 unite to form larger channels before acquiring their venous outlet. Fig. 

 3 illustrates this process in its inception. In this the root of the 

 mesentery contains many veins of much larger calibre formed by the 

 coalescence of the capillaries seen in the earlier stages. Even now, how- 



Fig. 2. — Transverse section through the renal anastomosis of the sub-car- 

 dinal veins of an embryo pig, 18 mm. long. In this the small capillaries seen 

 in Fig. 1 have become more numerous and of definitely larger calibre. A., 

 aorta ; R. A., renal anastomosis ; W. B., Wolffian body ; M. C, mesenteric capil- 

 laries ; Mes., mesentery ; G. A., genital anlage. 



ever, the fusion is not at all marked. The openings into the sub-cardinal 

 veins are much larger than at 17 mm. (Fig. 1), and they are still 

 numerous and quite distinct. 



In all these stages, as well as in the succeeding ones, both before and 

 after the complete separation of the sac, conclusive evidence for its 

 venous origin is derived from a special study of the sections through 

 the region of the meso-nephric arteries. Fig. 6 is taken from a section 

 through this region in an embryo of 32 mm. In this the early mesen- 

 teric sac is seen occupying the area between these arteries; it is com- 



