TAIL-KKGION OF rOLlSTOTFE:\[A (p.l )KLLOSTO.M a) STOUTl. o39 
vein becomes lioueycombecl witli mesenchymal s])aces and 
cavities of the various stao-es described al)Ove, beg-inninor 
anteriorly and gradually occurring* more posteriorly. By 
the coalescence of the larger cavities with each other and the 
vein, through the disintegration of the walls separating them, 
the caudal hearts are excavated out of the mesenchyme, 
mainly from above and below the anterior portion of the two 
branches of the caudal vein, and the caudal hearts for the 
most part become lined Avith endothelium derived from the 
flattening of the boundary mesenchymal cells of the larger 
cavities, and not from the migration of cells originating from 
p r e - f o r m e d e n d 0 1 b e 1 i u m . 
It Avas shoAvii beyond a reasonable doubt that the isolated 
mesenchymal spaces Avere not artifacts, and that they, 
as Avell as the larger connected mesenchymal cavities, fre- 
quently occur mesad of the tAvo branches of the caudal vein — 
a region never penetrated by capillaries or iutersegmental 
veins. 
In the 20 mm. series A, reconstruction 7, it Avas demon- 
strated that it Avould be possible for the proximal ends of 
three so-called dorsal iutersegmental veins to take a minor 
part in the formation of the caudal heart. It should, hoAv- 
ever, be stated that no trace or remains of these embryonic 
vessels Avere found in the 20 mm. series B or in any of the 
later series until the 85 mm. stage Avas reached. Conse- 
quently there Avould be just as much reason for advocating 
that the border cells of these embryonic vessels disintegrated 
or possibly reverted back to mesenchyme, from Avhich their 
cells are hardly distinguishable, as to maintain that in some 
Avay they contributed to the formation of the caudal heart. 
Granting, then, that they did take part in the formation of 
the Polistotrema caudal heart, it could only be a minor 
factor, and not the main process as is held by LeAvis, Baetjer, 
Huntington and Miller for the lymph-sacs of mammals, birds 
and reptiles. 
The formation of the caudal hearts as I have interpreted 
them in Polistotrema is almost identical to Avhat Stromsten 
