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 precardinals 
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 me- 
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 Figure 2. 
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- 
== =j- = PR'CRO 9. 
==--f-or Cuv 
=j- -Pcro s 
Fra. 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. Cuv., Cuvierian duct; ms'nph., mesonephros; p’erd. s., 
left postcardinal ; pr’crd. s., left precardinal; sn. vn., venous sinus; vn. atc., accessory 
vein ; vn. cor., coronary vein. 
