284 The Development of the Postcaval Vein in Birds 
sent actual reconstructions and are not in any sense diagrammatic. The 
subcardinal system throughout the series of illustrations is marked with 
crosses in order to facilitate the tracing of that portion of the postcava 
through the successive stages of development up to the adult condition. 
The subeardinal veins in the sparrow and chick are first found as a 
series of unconnected vessels or islands situated on each side between the 
mesonephros and the aorta 
(Fig. 1, V.se.dands). This 
is at a very early stage, be- 
tween the 60th and 70th 
hour in the chick, when the 
germ-layers are still flat ex- 
cept in the head region. In 
Fig, 2 (V. se. d. and s.),rene 
above mentioned islands are 
shown; on the right is seen 
a communication between 
the postcardinal and sub- 
cardinal. These islands sub- 
sequently fuse to form con- 
2 Fig. 1. Cross-section of averv voung sparrowem-_. e E 
bryo, corresponding to a chick 60-70 hours: taken at tinuous vessels in which the 
the level of the A.omphalomesenterica. Theasterisk _ . r 
(#) in Fig. 2 indicates the level of the above section. bilateral symmetry is com- 
< about 75. Ke x 
plete. This fusion, how- 
ever, may not be accomplished until after the proximal part of the post- 
cava is developed. 
The development of the subcardinals seems to correspond to the devel- 
opment of the mesonephros: as the mesonephros becomes larger the veins 
2? LETTERING OF ALL TEXT FiGurEs.—In the abbreviations d and s always refer to 
dextra and sinistra respectively. 
a. (Fig. 6), mesonephric veins. <A. i. (d ands), A. iliaca. Ao., aorta. A. o. m., A. 
omphalomesenterica. A. se. (d and s), A. sciatica. A. u. (a and s), A. umbilicalis. 
b. (Fig. 5), vessels enclosed within ventral side of mesonephros. c¢., cranial portion 
of the right subcardinal vein. d. (Fig. 7), anastomosis between posterior ends of 
postcardinals. JD. @. (d ands), ductus Cuvieri. D. V., ductus venosus. ki. (Fig. 
8), kidney. m., mesonephros. s. g. (Fig. 8), sexual gland. U7., ureter. V. c. (Fig. 
), V. coceygomesenterica. V.c. i., V. cava inferior (V. postcava). V.c. i. H. (Fig. 
), V. posteava, pars hepatica. V.c. i. SC. (Fig. 7), V. postcava, pars subcardinalls. 
V. c. p. (d and s), V. cardinalis posterior. V.c. p. (d and s) (V. i. i.), V. cardinalis. 
posterior (V. iliaca interna.) Vv. g., Vv. genitaliae. V.h. 7., V. hepatica revehens. 
a 
‘ 
a 
( 
V. i. c. d. (Fig. 9), V. iliaca communis. VJ. i. e. (d and s), V. iliaca externa. V. 7. U. 
(d and s), V. intervertebralis lumbalis. V. 7. m. (d ands), V. renalis magna. VJV.s. s., 
V. sciatica. V. sc. (d ands), V. subcardinalis. Vw. sr. s., Vv. suprarenales, V. w. 
(d and s), V. umbilicalis. W. d., Wolffian duct. «x. (Fig. 10), dorsal portion of loop- 
around umbilical artery. 
