336 
Wlf.LIAM V. ALLEN. 
region been examined^ these two chains of more or less con- 
nected mesenchymal cavities might readily have been taken 
for ont-buddings from the vein and the caudal heart, and the 
caudal heart been described as being formed from sprouts 
from the caudal vein. 
The endothelium and connective-tissue layers were found 
to be in about the same state as in the previous 25 mm. 
embi-yo and no intersegmental veins terminated in the left 
caudal heart. Immediately below the anterior end of the 
left caudal heart the two anterior mesenchymal cavities of the 
left embryonic ventral veno-lymphatic trunk (Fig. 10, 1. v. v. 
( 1 ), ( 2 )) li^d united, forming a large cavity, which, as in the 
previous series, had gained connection with the anterior 
ventral corner of the heart, and about the orifice there was 
a slight massing of the mesenchyme (Fig. 15, val. q)), possibly 
indicative of a very early stage in the formation of the valves 
guarding this opening. Likewise the third and fourth 
cavities of the embryonic ventral trunk (1. v. v. ( 3 ), ( 4 )) have 
coalesced and the latter has joined the heart, thereby estab- 
lishing the second or posterior communication between the 
ventral veno-lymphatic trunk and the left caudal heart. 
58 AXn 60 MM. POLISTOTEEMA SeRIES. 
As regards the caudal hearts in these two series, they 
appear to be practically in the same stage, and since the 
general preservation of the 60 mm. series is better, the des- 
cription will be taken almost entirely from this series. 
Except for a general dorsal expansion and a small caudal 
addition, the caudal hearts of the 60 mm. series have not 
increased in size relatively over the 27 mm. heart, but con- 
sidering the general histogenesis of the organ, marked 
progress has occurred. The posterior embryonic condition 
referred to above is in the posterior dorsal end of the heart, 
where both isolated and connected mesenchymal cavities 
(Fig. 8, c. and C.) are present. Xot only has the heart gained 
anterior and posterior connections with the ventral veno- 
