322 
WILLIAM F. ALLEX. 
Ill a very much decomposed embryo of 15 mm. the caudal’ 
artery and vein were present, but in a similar series of 10 mm. 
there was no trace of them. 
As in the adult the caudal artery and vein in the 20 mm. 
embryo separated into right and left branches above a 
thickening- of the mesenchyme that is forming- the lateral 
processes of the median ventral cartilaginous bar. Each fork 
of the caudal artery (Figs. 5, 6, and 7, B. and L. Can. A.) 
passes caudad immediately under and to the side of the 
notochord, at first along the side and further caudad above 
the level of the embryonic median ventral cartilaginous bar. 
The two caudal veins (Figs. 5, 6, and 7, B. and L. Gau. V.) 
follow at first obliquely ventrad across and then caudad along 
the ventro-lateral surface of the embryonic median ventral 
bar to a point in the 20 mm. series B (Fig. 6), 150 microns 
from the tip of the spinal cord or 290 microns from the tip of 
the tail. Here on the left side, and about the same place on 
the right side, the caudal vein and artery bend toward each 
other and unite, forming one continuous cross vessel on 
either side near the tip of the tail. Fig. 6 shows that there 
is in the anastomosing- portion of the caudal artery and vein 
several large masses of mesenchyme (P.)- These partitions 
were also found throughout the entire posterior portion of the 
caudal vein, including the region of the future caudal heart, 
of not only this 20 mm. series, but in another, series A, and 
in the 25 mm. series (Figs. 7 and 9, P.). This means, then, that 
this portion of the vein contains the remains of large 
mesenchymal cavity partitions that have not disintegrated at 
this stage. In the 25 and 27 mm. embryos the caudal artery 
and vein, on either side of the tail, were likewise found to be 
in direct connection a little ceplmlad of the ending of the 
notochord, but in all of the later stages this early connection 
is lost, and the caudal artery and vein extend farther caudad 
and break up or receive small branches coming- from, or going 
to, the caudal fin. 
Laterad and ventrad of the points of union of the caudal 
artery and vein in the 20 mm. series there is nothing- but 
