Mesenteric Sac and Thoracic Dnct in Embryo Pig 307 



velopment of the Lymphatic System in Eabbits," he states that ^a 

 portion of the sub-cardinal veins seems to become detached from the 

 rest to form lymphatics," and "that some lymphatics in the mesentery 

 accompanying the superior mesenteric and gastric veins may have arisen 

 as branches of these veins." In his conclusion it is stated that "similar 

 though smaller sacs than the jugular sac arise from the sub-cardinal and 

 mesenteric veins at a slightly later date." 



Thus, though the probable early venous connection of this sac is men- 

 tioned, no definite account is given of the time and method of origin, 

 nor of its differentiation from the venous system and subsequent con- 

 nection with the l}Tnphatics dorsal to the aorta — the thoracic duct 

 proper. To trace the origin and development of this sac, serial sections 

 were made of pig embryos in which the blood-vessels had been injected, 

 the embryos ranging in size from 16 mm. to 30 mm.^ In an embryo of 

 16 mm. there is as yet no evidence of the blood capillaries which later 

 form this sac, whereas at 30 mm. the sac has been completely differ- 

 entiated from the venous system and is abundantly connected with the 

 thoracic duct by large channels on each side of the aorta. This is well 

 shown in Fig. 9, which is taken from an embryo of 30 mm. ; the aorta 

 is seen suspended, as it were, in lymph. 



Cross sections through this sac in an embryo 22 mm. long just after 

 its differentiation, show shreds of tissue extending into the lumen and 

 very irregular margins, presenting a picture highly suggestive of tlie 

 fusion of many small vessels (see Fig. 7). The early stages were then 

 cut to ascertain whether it was possibly formed by the coalescence of 

 numerous capillaries and thus of direct venous origin, as had already 

 been shown for the cervical lymphatics. This was found to be the case. 



At 16 mm. no vessels exist in this region and the mesenteric attach- 

 ment is a homogeneous network of embryonic connective tissue, while 

 at 17 mm., the next stage examined, a very few small capillaries have 

 appeared (Fig. 1). These are shown in Fig. 1 as small, injected vessels, 

 lying in the root of the mesentery, just ventral to the renal anastomosis 

 of the sub-cardinal veins. Their course is extremely short, running 

 only the length of a few sections, and parallel to the long axis of the 



"It has been found by H. Mc.L. Evans that all injections should be made 

 through the umbilical artery, whether the embryos are dead or alive, since 

 venous injections are never so complete and are much more likely to cause 

 extravasations which are particularly prone to occur in this region. If the 

 embryo has only recently died, the heart will usually commence beating again 

 as soon as the injection mass reaches it. 



