TAIL-KEGIOX OF POF18TOTUEMA (bDELLOSTOMa) STOETI. 329 
■extreme distal end is shown, which is the region of future 
growth, and the manner of growth by the breaking down of 
the mesenchymal cell processes is evident from this section. 
Opposite line 12 in the reconstruction 6; the above cavity 
(C.) is shown to have attained anterior connection with the 
left caudal vein. 
Two dorsal connecting mesenchymal cavities (C) in the 
reconstruction 7 are typical examples illustrating a median 
stage of the development of a larger mesenchymal cavity that 
later becomes a portion of a caudal heart. These cavities 
simply represent one of the earlier connected mesenchymal 
spaces described in the previous paragraph as having united 
with one or more distal isolated mesenchymal spaces, thereby 
considerably increasing the size of the former. It might be 
surmised from an examination of the lateral reconstruction 7 
that these spaces are developing intersegmental veins or 
capillaries, but their position in transverse section is shown to 
be too far median for them. In fact similar cavities are found 
median of the caudal vein — a region never penetrated by 
capillaries or intersegmental veins. 
The third stage of development of the large connected 
mesenchymal cavities that contribute to the formation of the 
caudal hearts is simply a more advanced state of the middle 
stage previously described. It might be said to consist of 
several isolated mesenchymal spaces that have united with a 
larger connected mesenchymal space at either or both ends, 
and which brings these cavities in direct communication with 
the right or left caudal vein. Such a stage as this, or even a 
more advanced one, where all of the mesenchymal spaces 
have coalesced in one large cavity which is in direct connec- 
tion at both ends with the caudal vein, are shown in the 
reconstructions 6 and 7. In Fig. 13 one of these cavities (C.) 
is shown in section opening into the left caudal vein, and in 
Fig. 12 two of these cavities (C.) appear ventrad of the right 
and left caudal vein. Since they are cut through different 
planes they will illustrate very well two different areas of one 
of the large cavities in the third stage of development. In 
