TAIL-KEGION OF FOLISTOTREMA (eDEELOSTOMA) STOUTf. 311 
blastoderm, to form the omphalo-mesenteric vessels, and their 
endothelial walls are formed by a flattening of certain of the 
mesenchymal cells; while in a somewhat similar manner some 
of the enclosed mesenchymal cells are transformed by enlarge- 
ment and rounding into red corpuscles. Prof. Scammon, in 
his recent work on Squalus, clearly shows from careful recon- 
structions that the anterior cardinals in this shark are laid 
down as isolated vessels before any communication is estab- 
lished with the venous part of the heart, and that this connec- 
tion does not occur until a much later stage. In an early 
stage he finds a rather strong cephalic arterial connection, 
which he informs me likely gives rise to the anterior portion 
of this vein, but he is of the opinion that tlie posterior portion 
has an independent origin (probably from the union of certain 
mesenchymal spaces). 
On account of the resemblance of the lymphatics to venous 
sinuses in Polistotrema I shall describe them as veno-lym- 
phatics, which is indicative of a more generalized type, 
approaching the more primitive venous stage. It will be 
apparent that this use of the term differs from that made 
when it is applied to lymphatics arising ontogenetically from 
veins. 
Notwithstanding the fact that the caudal heart and some 
of the caudal vessels of Myxine^ and Polistotrema have 
been accurately described in the adult by Retziiis, Klinckow- 
strdm, Greene, and Favaro, it has seemed advisable for the 
sake of comparison to include here a brief description of these 
vessels as they appear in the adult from dissection and from 
serial transverse sections of a 20 cm. adult and an 85 mm. 
embryo that had assumed practically adult conditions. 
II. Yeno-Lymphatics in the Adult Polistotrema. 
Caudal Hearts (Figs. 1 and 4, i?. and L. Catc.H .). — Like 
Anguilla (eel), the caudal hearts are paired pulsating hearts 
' A complete bibliography will be furnished in the next communica- 
tion. 
