146 EDWARD MCCRADY, JR. 



in section D become very large, and the principal flow of blood 

 from the posterior part of the body is diverted ventrad at this 

 point into the subcardinals. These changes constitute the first 

 steps in the formation of the postcava, which will be discussed 

 further in connection with stage 33. 



The thoracic duct and the. jugular lymph sacs. It was men- 

 tioned in the previous section that the earliest anlagen of the 

 thoracic duct are mesenchymal spaces not associated with 

 veins, but that these later coalesce with a second set which are 

 formed around collapsing venules. Zimmermann has pointed 

 out that the first perivascular lymphatic anlagen of the thora- 

 cic duct arise in stage 32 (16174 and 17127). They are located 

 in the mediastinum between the levels of the second and 

 seventh thoracic vertebrae. " Caudal to these levels," he says, 

 ''there are no signs of thoracic duct anlagen in Normal Stage 

 32. Cephalad, only the ill-defined mesenchymal clefts of the 

 preaortic mesoderm are found." 



The degenerating venules around which these first peri- 

 vascular thoracic duct anlagen form, are tributaries of the 

 thoracic portion of the postcardinal veins. Almost immedi- 

 ately after they begin to form, these peri-intimal spaces run 

 together and establish a lymphatic plexus. The fusion of the 

 separate anlagen occurs first at the levels of the fifth and sixth 

 thoracic segments, and later spreads from this point in both 

 directions longitudinally. As the process extends cephalad 

 the mesenchymal clefts formed in stage 31 are met and in- 

 corporated in the lymphatic plexus. Between the most an- 

 terior of these and the anlagen of the jugular lymph sac there 

 is still a long gap in which no lymphatic anlagen are found. 



The jugular lymph sacs are at a similar stage of develop- 

 ment. The remnants of the degenerating paracardinal venous 

 plexus have pinched off and are completely detached from the 

 venous system. The perivascular lymphatic anlagen are 

 coalescing to form a continuous lymphatic plexus not in com- 



Fig. 47 Keconstruetions and sections from stage 32. The central reconstruc- 

 tion is from 16173 except for the membranous labyrinth which is added from 

 17127. H and G are from a wax model made by Dr. C. H. Heuser and now 

 in The Wistar Institute. A, E and F are from 17127. The other sections are from 

 16174. J shows the real size of the embryo and allantois. 



