FATE OF PRIMARY LYMPH-SACS IN ABDOMINAL REGION OF PIG, ETC. 21 



LYMPHATICS ARISING VENTRAL TO THE AORTA. 



RETROPERITONEAL SAC. 



In the smaller embryos, beginning at about 23 mm., there is a primary lym- 

 phatic sac the mesenteric (Lewis, 1906) or retroperitoneal sac, as it was termed 

 later (Baetjer, 1908) situated outside of the peritoneal cavity. This sac lies 

 between the paired Wolffian bodies and gonads, just ventral to the point of renal 

 anastomosis of the subcardinal veins, and extends from the region of the superior 

 mesenteric artery to a point near the bifurcation of the aorta (fig. 1). Up to about 

 7 to 8 cm. it is a complete sac spreading sheet-like between the Wolffian bodies 

 (Heuer, 1909, figs. 3 and 4), and later between the kidneys. 



Lewis (1906) describes the sac in rabbit embryos of 21 mm. as situated at the 

 root of the mesentery, just ventral to the point of renal anastomosis of the sub- 

 cardinal veins. Baetjer (1908) has shown that it has its origin in numerous venous 

 capillaries which, by increase in number and by fusion, form a sac which has definite 

 connections with the subcardinal veins. There follows a gradual obliteration of 

 these venous connections and for a short period the sac is an isolated structure with 

 irregular margins. By the stage of 25 mm. it has gradually developed connections 

 with the rest of the lymphatic system by an upgrowth of small lymphatic capillaries 

 along the margin of the aorta. Heuer (1909) describes three main connections with 

 the lymphatics dorsal to the aorta : 



(1) There are several afferent vessels which arise from the mesial trunk as it 

 leaves the anterior end of the sac and course ventro-dorsad and slightly anterior to 

 enter the thoracic duct (fig. 2, Anas. ant.). 



(2) The main efferent connection is a stout trunk (fig. 2, Anas, maj.) opposite 

 the anterior pole of the kidney, between the cisterna chyli and the anterior end of 

 the sac. 



(3) A third connection is made from its posterior portion to the caudal part 

 or fan of the iliac sacs. Also, along the renal portion of the aorta there are numerous 

 anastomoses with the lymphatics dorso-lateral to the aorta. 



Anterior portion of sac. This large sac ventral to the aorta may be divided 

 in the region of the hilum of the kidney into an anterior and a posterior portion 

 (fig. 1). The anterior portion gives rise to two main trunks, the large anterior or 

 coeliac trunk and the small posterior or superior mesenteric trunk. Figure 1 shows 

 the ventral or retroperitoneal sac in a pig embryo 7.8 cm., after it had begun to 

 split into a plexus of lymphatic vessels, the forerunners of its transformation into 

 lymph-glands. In the pig it is at this stage a very extensive plexus, in contrast with 

 its earlier sac-like form (Heuer's fig. 4), and its main portions are seen to lie on 

 either side of the aorta, where they form a coarse network of channels. It will be 

 noted that the fold of mesentery and the cceliac trunk have been cut and the 

 stomach turned aside to expose the lymphatics from the anterior end of the sac. 

 At this stage the kidneys have developed into large structures and the Wolffian 

 bodies have begun to degenerate and are pushed to the side. The artist has clearly 

 shown the relationship of the various parts of the reproductive system. 



