TAlL-ltEGlOX OF POLISTOTREMA (ni)ELEOSTOM a) STOUTI. 327 
14) to be in very early stages of formation. In fact, the 
•caiulal vein itselt‘ may not have received its maximum size 
in this region any more than it has farther cephalad or 
camlad, but since in the adult the caudal hearts are nothing 
more than large expansions of the anterior portion of the 
right and left branches of the caudal veiu, it is impossible in 
the embryo to discriminate or define any boundaries between 
the enlargement of the caudal vein such as has been described 
both anterior and posterior of the caudal hearts from the 
normal development of the caudal hearts, for both take place 
in exactly the same manner. 
A glance at the embryonic caudal heart in the reconstruc- 
tions 6 and 7 demonstrates that the original left caudal vein 
{L. Cau. V.) is bounded, more or less on all sides, by certain 
mesenchymal spaces, from the region of the embryonic 
lateral processes of the median ventral cartilaginous bar to a 
point designated by Z., denoting the posterior extremity of 
the caudal heart as determined by its relation to the spinal 
nerves. These spaces can readily be separated into two 
distinct types: first, mesenchymal spaces (C.) that have gained 
connection with the caudal vein, and second, mesenchymal 
spaces (c.) that have not at this stage obtained connection wnth 
the caudal vein. 
For the sake of maintaining a logical order the small 
isolated mesenchymal cavities (c.), that have not at this stage 
gained connection with the caudal vein, will be considered 
first. As will be shown later they are undoubtedly the first 
cavities to appear. In the two reconstructions they are found 
everywhere where the heart is in the process of formation, 
and in the extreme posterior end of the embryonic caudal 
heart in reconstruction (3, which is the region of the heart 
last to form, they were the onl}^ cavities observed. One of 
these cavities is shown in sections 11 and 13; the former is 
taken through an isolated cavity in the posterior end of the 
embryonic caudal heart and the latter is from the anterior 
end. In both cases these cavities are small, but well-defined, 
not artifacts, and are bounded simply by the adjacent 
