334 
WILLIA.M F. ALLFX. 
25 AND 27 MM. POLISTOTREMA SeRIES. 
A glance at fig. 9 will demonstrate that the left embryonic 
caudal heart [l.cau.li.) of the 25 mm. embryo has made 
considerable progress over both of the 20 mm. embryos. 
The anterior ventral region has been completely excavated, 
as has also the anterior dorsal region, except in the latter 
there is left traces of the mesenchymal partition that formally 
separated two of the larger cavities from each other or a 
cavity from the vein. The middle and posterior portions of 
the heart have not received their maximum dorsal growth at 
present, and the mesenchyme above and to either side of the 
origiual left caudal vein is invested by a number of large 
connected mesenchymal cavities and several isolated cavities, 
while the extreme posterior end of the caudal heart, namely, 
that portion located in front of the line Z., is made up solely of 
the original caudal vein surrounded on all sides by isolated 
mesenchymal spaces (c.). 
Anteriorly in the dorsal two thirds of the 25 mm. embryonic 
caudal heart the endothelium is much further differentiated 
than in the previous series, which means that its cells are some- 
what more flattened, and their distal processes that formally 
extended out into the mesenchyme are lost, but ventrad, 
throughout the entire heart the border-cells are nearly round, 
very abundant, with almost no processes, and in a state where 
they could be easily moved. It would be impossible to dis- 
criminate between the ventral border-cells of the heart and 
the undifferentiated mesenchyme more distad. 
There are no intersegmental veins emptying into the 25 mm. 
caudal heart unless the extreme proximal or caudal vein 
portion is considered as part of the heart ; if so, the caudal 
heart would receive a dorsal and a ventral intersegmental 
vein (Fig. 9, hits. V.). It will be seen that the first or anterior 
mesenchymal cavity of the left embryonic ventral veno-lym- 
phatic trunk (Fig. 9, /. v. v. q)) has gained connection with the 
anterior ventral corner of the left caudal heart, but no valves 
have appeared at the orifice (0.). Three other mesenchymal 
