324 
AVELLIAM F. ALLEX. 
ineseiicliymal cavities, wliicli for tlie most part have gained 
connection with the vein and with eacli other. They are lined 
with an undifferentiated mesenchymal endothelium, which cells 
in many cases have lateral processes that cross these cavities as 
well as line them. In a later stage, 27 mm., the mesenchymal 
walls between these cavities and the caudal vein have broken 
down, thereby greatly increasing the lateral diameter of the 
vein. F rom these cavities both in the 20 and the 27 mm. series 
short dosal and ventral intersegniental branches were given 
off ; these for the most part ended peripherally in more or 
less of a cord of cells, indicating that the origin of the inter- 
segmental veins is from sprouts off from the caudal vein. 
Even in these small embryos the intersegniental arteries have 
attained the course ali-eady described for the adult. In the 
27 mm. embryo the median branch to the spinal cord is more 
clearly detined than in the adult, and in both series the inter- 
segmental arteries travelled much further laterad before 
dividing than they did in the adult. 
In the region of the caudal heart and behind the caudal 
heart in the 20 mm. series A, which is not as far advanced as 
series B, all of the intersegniental vessels are in an embryonic 
condition. The caudal intersegniental vessels are more 
numerous than intersegniental vessels should be ; those in the 
caudal heart region appear to be about twice the normal 
number and to increase as you go caudad. Xowhere in this 
region do you ffnd a typical intersegniental artery arising 
from the lateral surface of the caudal artery and dividing into 
dorsal and ventral branches, but rather each branch takes its 
origin from dorsal or ventral surface oE the right or left caudal 
artery (Fig. 7, Infs. J.) and then passes latero-dorsad orlatero- 
veiitrad. In the region of the caudal heart there are at lea^^t 
three latero-dorsal outpocketings, which have every evidence 
oE being embryonic intersegniental veins (Fig. 7, hits. F.). 
They at first passed laterad, then dorsad, and upon reaching 
a height about on a level with the leEt caudal artery, but con- 
siderably laterad of it, they end blindly. The second of these 
embryonic veins in the heart region (Figs. 7 and 14, hits. F.) 
