TAIL-REGIOX OF POLISTOTKEMA (llDEl.EOSTOM a) 8TOUTI. 357 
space (I. V. i\). In some respects series A is more embryonic than series 
B. X 200. 
Fig. 8. — G-raphic reconstruction of the left caiulal artery, vein, heart, 
and the ventral and ha3inal venodymphatic systems of a 00 mm. 
Polistotrema. Posteriorly the candal heart is shown to be still in the 
process of formation, but in the main it l>as assumed adult conditions. 
Likewise the ventral venodymphatic trunk has attained practically adult 
conditions. Two connections are established with the left caudal heart, 
each of which is guarded by a pair of lateral valves, and since this is a 
median reconstruction, it passes between them, so that they are not 
shown in this figure. In the region immediately in front of the caudal 
hearts the longitudinal lia3tnal lymphatic trunks proper are practically 
the same as in the adult, but no trace of the ventral longitudinal hsemal 
lymphatic trunk has apppeared in this stage. X 50. 
Fig. 9. — Graphic reconstruction of a portion of the notochord, the 
caudal artery and the left caudal artery, the caudal vein and the left 
caudal vein, the embryonic caudal heart and the emlnyonic ventral 
venodymphatic trunk of a 25 mm. Polistotrema. It should be noted 
that the caudal heart is fairly well formed anteriorly, but posteriorly it 
is in an early stage of formation, showing very conclusively that it is a 
gradual process, taking place from cephalad to caudad. This process is 
discussed at length in the text. Considerable progress is also shown in 
connection with the development of the ventral venodymphatic trunk 
over the 20 mm. reconstructions. An identical mesenchymal cavity 
(/. V. V. ( 1 )) is shown, but it has increased in size and obtained connection 
with the embryonic caudal heart. Also additional mesenchymal spaces 
have put in their appearance in a direct line posterior to this, and in 
one place (o. ( 2 )) there is a collection of small cavities, indicating 
possibly the place where the second or posterior connection with the 
caudal heart will occur, x lOO. 
Fig. 10. — Identical reconstruction of the same structures of a 27 mm. 
Polistotrema as the previous reconstruction of the 25 mm. Polisto- 
treuia. It, however, shows considerable advancement, although from 
its centre to its posterior border it is in more or less embryonic con- 
dition. Two large mesenchymal cavities (0.) that have gained connec- 
tion with the caudal heart and vein are very conspicuous. From a 
lateral view the posterior might readily be taken for an intersegmental 
vein, or a degenerate intersegmental vein that was taking part in the 
formation of the caudal heart, but, as was shown in the text, its extreme 
median position is against this hypothesis. Note the marked and 
regular progress in the formation of the ventral veno-lymphatic trunk. 
Cavities {1. v. v. (d, ( 2 ) ) have coalesced in one long cavity and cavities 
{L V. V. ( 3 ), (i)) have also united, and both of these large cavities have 
