3oO 
WILLIAM F. ALL EX. 
tlie floor of tlie left caudal vein will be seen (Fig. 12, C.) one of 
tliese cavities opening into the vein ; observe that the orifice 
bears evidence of the breaking down of the partition 
separating the cavity from the vein. In tracing this cavity 
cephalad it was found to open into three smaller mesenchymal 
spaces almost identical to the three mesenchymal spaces ((7.) 
of another cavity of the third stage, shown below the right 
caudal vein in the same figure, and when followed further 
cephalad the mesesenchymal spaces terminated in a large 
cavity that emptied into the left caudal vein from below. A 
little anterior of the cavity described above there is another 
large connected cavity of the third stage, not shown in any 
of the transverse section figures, but almost identical to the 
cavity previously described. It has, however, one striking 
difference, namely, the process of the union of its mesen- 
chymal spaces has progressed further ; Avhile the mesenchymal 
walls separating these mesenchymal spaces in the first cavity 
were quite thick, resembling the condition found in the 
ventral cavity below the right caudal vein in Fig. 12; in this 
more advanced cavity they are reduced to two parallel 
mesenchymal cell processes, separating three median-lateral 
mesenchymal spaces from each other. These two remaining 
processes consist of the union of two ventral processes from 
two mesenchymal cells, now situated as border-cells on the 
dorsal .side of the future mesenchymal cavity with two 
similar dorsal processes from two mesenchymal cells located 
on the ventral border of this future mesenchymal cavity ; 
while from all of these mesenchymal cells there are median- 
lateral processes, which wdll help constitute a sort of un- 
differentiated endothelial lining for this cavity. It would 
require but the atrophy of these delicate protoplasmic pro- 
cesses to convert the three mesenchymal spaces into one large 
mesenchymal cavity that would be connected at both ends 
with the left caudal vein, and in like manner the disintegra- 
tion of the partition separating this cavity from the vein 
would result in the excavation of a considerable portion of 
the caudal heart in the ventro-cephalic region. 
