TAIL-REGION OF POLISTOTREMA (lDELLOSTOMA) STOUTI. 381 
Now the question might be asked, What becomes of the 
mesenchymal cells in the walls of tlie mesenchyme separating 
one space from another, as, for example, in Fig. 12, the ventral 
mesenchymal spaces [C.), when they coalesce to form one 
large cavity ? It is quite certain that some of these cells are 
becoming round and increasing in size pre]:>aratory to 
becoming red corpuscles, while the border cells are apparently 
flattening to a slight extent in order to become endotlielium 
for the caudal heart. It is doubtful, however, if all the cells 
take one or the other of these two courses. There remains, 
then, the processes of disintegration and migration as 
possibilities. 
In connection with the process described above of an 
isolated mesenchymal space becoming in a later stage con- 
nected with the caudal vein or becoming a connected mesen- 
chymal space as it was styled, the question might be asked. 
How do you know that this change actually occurs without 
being able to demonstrate it experimentally ? In reply it 
might be said that it would be absurd to consider these well- 
defined mesenchymal spaces as artifacts, for if artifacts, they 
would occur in the mesenchyme outside the limits of the 
developing caudal hearts, blood-vessels, and lymphatics. In 
the immediate region of the caudal heart they are found only 
in the region of the small connected mesenchymal spaces and 
the larger mesenchymal cavities that unite with each other 
and the caudal vein to form the caudal heart. At the orifices 
of a great many of the small connected mesenchymal spaces 
and the larger mesenchymal cavities there are left the remains 
of the partitions that once separated these spaces or cavities 
from the vein, which is evidence against the hypothesis that 
they arose as sprouts or out-pocketings from the vein, as is 
also the fact that these cavities are lined with mesenchyme 
and not differentiated endothelium. Xot many of these 
cavities have sufficient regularity to be the remains of inter- 
segmental veins. In fact, they occur mesad of the caudal 
vein, a region never traversed by intersegmental veins or 
capillaries. The isolated mesencli 3 nnal spaces and the more 
