470 David M. Davis. 
mesentery. Since the subcardinal takes so prominent a part in 
the synthesis of the cava, its own origin must be of interest. 
Lewis spoke of it in rabbits as the result of longitudinal anasto- 
moses between small veins entering the cardinal from the mes- 
entery and mesonephric tributaries. This must be disputed, at 
least in the pig, since in a number of early injected specimens, 
the writer has seen distinct subeardinals, having no tributaries 
whatsoever from the mesentery. These tributaries appear later. 
At the earliest stage, the mesonephros is seen filled with a net- 
work of capillaries springing from the cardinal vein; soon some 
of them ventral to the mesonephric arteries dilate and form the 
longitudinal channel of the subcardinal. The paper of Grafe, 
written of the chick’s mesonephros, bears eloquent witness on 
this point, even though he supposed some of the vessels to arise 
in situ. Thus we have the history of the caval channel arising 
everywhere from capillary anlagen. At later stages, numbers of 
channels are formed in the same way in the mesonephros. An 
example of them is a great ventro-lateral vein, appearing in the 
different drawings. It is seen in pigs as small as 7 mm., and draws 
blood from the periphery of the gland at its ventro-lateral angle. 
It retains connections with the anterior portion of the posterior 
cardinal vein until a very late stage, when, with the regression of 
the Wolffian body it disappears, taking no part in the formation 
of any important permanent vein. 
The manner of origin of the various trunks of the vascular 
systems has long interested embryologists, and twe sorts of expla- 
nation have been advanced: according to one idea, arteries and 
veins grow out as such to their end territories, and it will be 
recalled in this connection that Hochstetter spoke of the vena 
cava as “growing down”’ into the region of the Wolffian body. 
According to the other idea of the development of the vascular 
system, arteries and veins exist originally in the form of simple 
capillaries, usually in the form of a typical plexus. rom this 
netlike or plexiform condition the trunks of the adult are differ- 
entiated. The latter idea, suggested by Aeby and Baader, has 
recently been proven to be the correct one in the history of vari- 
ous vessels, Evans having been able to show that the very largest 
