A. M. Miller _ 285 
increase in length and calibre, and in the earlier stages they are found 
to extend the full length of this organ. A number of direct communi- 
cations between the posteardinal 
and subcardinal veins are pres- 
ent, which pass between the me- 
sonephros and the aorta. In Fig. 
4 (sparrow, corresponding to a 
chick of 90 hours) three of these 
communications are seen on each 
side. So long as the mesone- 
phros is small these anastomoses 
are of considerable size and im- 
portance, but as this organ de- 
velops they are broken up in the 
mesonephric circulation, and 
cease to exist as_ well-defined 
vessels. However, the subcar- 
dinals may remain in communica- 
tion, at their anterior ends, with 
the postcardinals even after the 
postcava appears (Iigs. 4 and 5). 
In the chick the writer has ob- 
served other direct connections 
between the subcardinals and 
postcardinals in the form of well- 
defined vessels enclosed within 
the ventral part of the mesone- 
phros. These vessels are shown 
in Fig. 5 (b) (chick, 90 hours) 
lying at the side of the subcar- 
dinals and ventral to the postcar- 
dinals. No particular importance 
is to be attached to them since 
they, like the other anastomoses 
between the postcardinals and 
subeardinals mentioned above, 
are subsequently broken up in 
the circulation through the me- 
Fig.2. Reconstruction of the venous system 
of a very young sparrow embryo, correspond- 
ing to a chick of 60-70 hours. Ventral view. 
<1. (Explanation of lettering on page 284.) 
sonephros. Soon after their first appearance the subecardinals receive 
blood from the mesonephros through a number of small branches which 
enlarge as the above organ enlarges. In the figures of the early stages 
these branches have been omitted to avoid confusion. 
