TAIL-KEG lOX OF POLISTOTRE^rA (lDELLOSTOMa) STOUTI. 341 
l\’mpliatic trunk. At this stage it has not obtained connec- 
tion with the anterior end of the embryonic caudal heart, 
although transverse section 13 demonstrates that the mesen- 
chyme between the two is becoming vacuolated, indicating 
the place where the first or anterior communication will occur. 
Still further caudad in reconstruction 6 there is another 
isolated mesenchymal cavity {v. v.), which occupies a position 
too veil trad to take part in the formation of the left caudal 
heart, and moi-e than likely represents one of the more 
posterior cavities of the chain of cavities that will later unite 
in forming the posterior portion of the ventral trunk. The 
20 mm. series A portrays a still earlier stage of this most 
anterior cavity of the ventral trunk, which is nothing more 
than a small vacuole in the mesenchyme (Fig. 7, 1. v.v.^d). 
The left ventral veno-lymphatic trunk in the 25 mm. series 
is shown in reconstruction 9 to consist of a longitudinal chain 
of at least four mesenchymal cavities {1. v. v. (d_( 4 )). The 
first cavity (/. v. v., (d ) is of considerable size, and is 
probably derived from the union of several smaller mesen- 
chymal spaces. As in the 20 mm. series, it not only passed 
cephalad under the embryonic left lateral process of the 
median cartilaginous bar, but extended further cephalad to a 
point 30 mm. in advance of union of the two branches of the 
caudal vein and considerably past the point of bifurcation of 
the caudal artery, while posteriorly it had gained connection 
with the anterior ventral corner of the left caudal heart, 
probably through coming in contact with the heart and the 
breaking-down of the wall separating them. Of the three 
remaining cavities, the first or second in the chain [1. v. v. (O)) 
is still isolated, while the two last have united in a single 
cavity, and tlie mesenchyme above this cavity is more or less 
vacuolated, preparatory to establishing the second or posterior 
connection (o. ( 2 )} with the left caudal heart. 
In the 27 mm. series, reconstruction 10, the embryonic 
ventral veno-lymphatic trunk is represented by a longitudinal 
chain of at least five mesenchymal cavities. The first and 
-second cavities have coalesced in one, which not only com- 
VOL. 59, PART 2. NEW SERIES. 22 
