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Walter A. Baetjer 



embryo. Toward the lower end they turn dorsalward, emptying finally 

 into the renal anastomosis by distinct openings, as shown also in Fig. 1. 

 This is quite in contrast with their mode of venous connection in the 

 later stages in which there is definite fusion into much larger channels 

 before their final termination (Fig. 3). Through the stage of 18 and 

 up to 19 mm. there is a gradual increase in these small mesenteric veins; 



Fig. 1. — Transverse section through the renal anastomosis of the sub-car- 

 dinal veins of an embryo pig, 17 mm. long, showing the small veins in the 

 root of the mesentery, which are the anlage of the mesenteric sac A., aorta ; 

 R. A., renal anastomosis ; W. B., Wolffian body ; M. C, mesenteric capillaries ; 

 Mes., mesentery; O. A., genital anlage. 



they become more numerous and of definitely larger calibre, while at 

 the same time there is an increase in their length (Fig. 2). Fig. 2, 

 taken from an embryo of 18 mm., shows well the initial steps in this 

 process. The region occupied by the minute capillaries in the embryo 

 of 17 mm. (Fig. 1) is seen to contain many more veins, all of which are 

 definitely larger than those seen in the preceding section. A later and 



