TAIL-REGION OF POLISTOTREMA (bDELLOSTOMA) STOUTI. 32a. 
undifferentiated mesenchyme. No trace of the embryonic 
myotomes or lateral sinus mesenchymal spaces occurs until a 
section 210 microns cephalad of the tip of the spinal cord is 
reached. 
Intersegmental Blood-vessels. — As previously stated 
in the adult under the head of the intersegmental or hasmah 
lymphatics there are twice as many intersegmental lymphatics 
as there are arteries or veins — that is^ the arteries alternate 
with the veins in traversing the septa between the myotomes;. 
while each septum would have a lymphatic vessel, it would 
have either an artery or vein, not both. A typical interseo-- 
mental artery upon leaving the lateral side of the catidah 
artery almost immediately divides into a dorsal and ventrnl 
branch. In the course of the dorsal branch, it gives off lateral 
branches to the myotomes and a median branch to the neural 
canal; while the ventral branch passes ventrad along the 
inner surface of the septum between two myotomes, supplying 
each. The course of the intersegmental veins is practically 
the same as the arteries, except that the dorsal and ventral 
veins terminate separately at different levels on the lateral 
surface of the caudal vein instead of first uniting in a common 
stem. The veins, then, are apparently in a more primitive 
condition than the arteries. 
It is not my intention to take up the development of these 
vessels only in so far as they are concerned in the formation 
of the caudal hearts, but in order to get a true conception or 
typical state of these vessels in the embryo it is necessary to 
examine them in front of the point of bifurcation of the caudal 
vessels. 
In the 20 mm. series B, it will be seen at a glance that the 
intersegmental arteries are much further advanced than the 
veins, which have barely started. It will also be noted that 
the caudal artery and vein have assumed different shapes; 
the artery is very much depressed, having a very great lateral 
diameter and almost no dorso-ventral diameter, while the 
caudal vein is almost round. Throughout the whole length 
of the caudal vein there are on either side one or two- 
