TAIL-KEGIOX OF rOLTSTO'l’KE:\[A (uDELLOS'rOM a) STOUTI. 349 ' 
liypural bones and the tail muscles, thus discharging their 
contents into the caudal veins and functioning as the caudal 
hearts of Polistotrema. 
With the exception of some additions noted in connection 
with the hmmal lymphatics, the adult veno-lymphatics in the 
tail region were found to be about the same as Grreene and 
Favaro had described them. 
In my youngest stage the arterial system Avas well formed, 
Avhile the venous system Avas in a rather late embryonic state. 
In the region of the post-gut the caudal vein Avas in com- 
munication Avith the yolk-sac veins, and near the tip of the 
notochord a branch on either side anastomosed AAuth a cor- 
responding branch of the caudal artery. Throughout the vein 
there Avas everyAvhere the remains of mesenchymal cavity 
partitions that had not fully disintegrated at this stage. The 
caudal vein Avas described as growing in diameter by the 
addition to either side of certain isolated mesenchymal spaces, 
but strange to say, the intersegmental veins Avere formed as 
dorsal and \mntral sprouts from the lateral surface of the 
caudal vein. 
AVhile the early stages of the formation of the blood-vessels 
in Polistotrema Avere not to be had, considerable evidence 
was gathered in support of a hypothesis that I Avould like to 
propose for the origin of the blood-vessels, and Avhich may 
also hold good for the lymphatics, namely, that the larger 
longitudinal trunks Avhich folloAv the main axis of the embiyo 
are developed through the formation and coalescence of certain 
mesenchymal caAuties, AAdiile the intersegmental and smaller 
branches arise as outgroAA^ths from the Avail of the main trunks 
already formed. The process iuAmlA’ed in connection AAuth the 
larger trunks is identical to LaucastePs description of the 
formation of the blood-Amssels in the leach by a vacuolation 
of the mesenchyme; some of the cells by flattening are going 
to form the endothelium, and others, by becoming roufld and 
increasing in size, are transformed into red corpuscles. In 
connection AAuth the outgroAvths of the intersegmental vessels 
fiom the AA'alls of the main trunks it should be noted that all 
