PARKER AND TOZIER : POSTCARDINAL VEINS IN SWINE. 135 



duct of the corresponding side. The partial penetration of the meso- 

 nephros by the postcardinal might with equal propriety be described as 

 a partial intrusion of the mesonephros into the cavity of the vein, for 

 the loosely twisted nephridial tubules hang with such freedom into the 

 venous blood-spaces that they may with justice be said to occupy more 

 or less of the cavity of the vein itself. This peculiar suspension of the 

 tubules in the venous blood-spaces, a 

 condition which persists in the later 

 stages, has been recently noticed by 

 Minot ('98, p. 229). 



In an embryo whose greatest length 

 (measured as before described) was be- 

 tween twelve and thirteen millimeters 

 the veins just considered present the 

 condition shown in Figure 2. Except- 

 ing for differences of size, and slight 

 changes in outline, the precardiuals 

 are essentially the same as in the pre- 

 ceding stage. In the region of the ve- 

 nous sinus the hepatic opening, which 

 is now single, and the anterior opening, 

 which leads into the heart, have shifted 

 somewhat toward the right side of the 

 body, and the root of a coronary vein 

 (vn. cor.) has been formed. 



The postcardinals are very much 

 altered. With the growth of the rae- 

 sonephroi, they have become entirely 

 interrupted in the middle part of their 

 course ; their posterior portions, how- 

 ever, persist near the base of the hind 

 legs, finding an outlet toward the heart 



through the newly formed postcava, and their anterior portions now 

 begin at the anterior ends of the mesonephroi, and each extends to the 

 Cuvierian duct of its own side. The anterior portion of each postcar- 



FlGUBE 2. 



Fig. 2. Reconstruction on a frontal plane of the principal veins and parts of 

 the mesonephroi of an embryonic pig between twelve and thirteen millimeters long. 

 Ventral view. X 12. dt. Cxt'., Cuvierian duct; ms'nph., mesonephros; p'crd. s., 

 left postcardinal ; pr'crd. s., left precardinal ; sn. vn., venous sinus ; in. ace, accessory 

 vein; vn. cor., coronary vein. 



