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Walter A. Baetjer 



the same embryo, the sac is even more definitely formed. It also shows 

 that the sac is still injected from the vein. The drawing shows the 

 beginning of the mesonephric arteries arising from the aorta, the area 

 between them being occupied by the large channels still definitely con- 

 nected with the veins, whereas, in Pig. 6, from a section through the 



4 fS >?SWyA^is^^i/m^i%^:w^iif' ■n,A'^si^Ajt^/^hH 



Fig. 5. — Transverse section through the beginnings of the mesonephric 

 arteries in an embryo pig, 20 mm. long. The section shows the beginnings of 

 a definitely formed sac which, however, can still be traced into the veins. 

 The venous connection at this stage, however, is gradually becoming oblit- 

 erated. A., aorta ; P. C F., post-cardinal veins ; M. C, mesenteric capillaries ; 

 Mes., mesentery ; W. B., Wolffian body ; li. A., beginnings of mesonephric 

 arteries. 



same level in a 22 mm. embryo, the sac has lost almost all venous con- 

 nection and has become a definite part of the early lymphatic system. 



This active process of development continues between 20 and 21 mm. 

 until, at the latter stage, there is formed a definite sac with irregular 

 margins, due to the coalescence of numerous capillaries; and similar in 



