TAIL-UEGLON OF POLISTOTKEMA (bDELLOSTO^EA) STOFTI. 385 
cavities belonging to the left ventral trunk have also appeared 
in a row behind the first (Fig. 9, 1. v. v. ( 2 )-(i)) , ‘^^d above the 
third and fourth, which have coalesced, the mesenchyme is 
vacuolating, indicating the place (0.(2)) where the second or 
posterior communication wdth the heart is likely to occur. 
Reconstruction 10, which is from a 27 mm. embryo, shows 
a marked advance in the development of the caudal heart 
over the previous 25 mm. embryo as seen in reconstruction 
9. Anteriorly the caudal heart had reached practically adult 
conditions so far as relative size was concerned, but from its 
centre caudad it consisted of a number of very large connected 
mesenchymal cavities (c.), which have occurred and assumed 
lines of growth in two different directions. The most anterior 
path or chain of these mesenchymal cavities had assumed a 
general caudal direction from the dorsal central part of the 
heart ; while the posterior chain of the mesenchymal cavities 
had taken a dorsal course, considerably posterior, from the 
left caudal vein. A glance at Fig. 10 will show that the 
distal ends of these two different lines of growth of mesen- 
chymal spaces, for the most part connected, were about to 
anastomose, and when this is accomplished the general out- 
line of the heart will be completed, excepting that its dorsal 
boundaries will be expanded somewhat, and it will be added 
to posteriorly by the formation and union of a few additional 
mesenchymal spaces. These posterior isolated cavities were 
not present in this series, but were found in all of the later 
stages. As a result of the meeting and union of these two 
chains of mesenchymal cavities decribed above, there would be 
left in the centre an island of mesenchyme, identical to an 
anterior island (Fig. 10, P.) already formed, which in due time 
would be eliminated from the heart. From the lateral recon- 
struction 10, the posterior chain of mesenchymal cavities has 
the appearance of a developing intersegmental vein, but 
transverse sections reveal its position to be too far median — 
in fact it arises from the dorsal surface of the vein, and its 
course is dorsad close to the embryonic median ventral carti- 
laginous bar. Had not earlier stages of the caudal heart 
