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Walter A. Baetjer 



pletely differentiated from the neighboring veins which are filled either 

 with blood corpuscles or the injection mass. In the earlier stages, how- 

 ever, this area is studded with numerous small veins. It is thus evident 

 that this structure, which later becomes an integral part of the lymphatic 

 system, is represented in the earlier stages by a plexus of small veins. 



Fig. 3. — Transverse section through the renal anastomosis of the sub-car- 

 dinal veins of an embryo pig, 19 mm. long. The veins in the root of the 

 mesentery have fused to form the larger channels seen in this section. This 

 is the beginning of the processes of fusion which leads to the formation of 

 the mesenteric sac in the later stages. At this stage, however, these vessels 

 can all be traced definitely into the renal anastomosis. A., aorta; R. A., renal 

 anastomosis ; W. B., Wolffian body ; M. C, mesenteric capillaries ; Mes., 

 mesentery ; G. A., genital anlage. 



Up to this time, i. e., 19 mm., the increase both in the number of 

 capillaries and the amount of fusion, has been very gradual. From 

 now on, however, the development in both of these phases goes on with 

 great rapidity; in fact, the active process in the formation of the sac 

 may be said to begin here. Between 19 and 20 mm. the number, calibre 

 and lensrth of these vessels become markedly increased : there is much 



