352 
AVILMAM F. ALLEX. 
origin directly from veins, tliat they would hegin exactly as the 
veins did, namely, by the vacuolation of the original mesen- 
chyme. In one of the primitive vertebrates, Polisto t rema, 
I have described these vessels as veno-lymphatics, by which 
is meant a system of vessels that is more closely related ta 
the veins than the lymphatics of the higher vertebrata are. In 
a later communication I expect to show that the main factor 
in the construction of the veno-lymphatic system is the same 
as was described for the caudal hearts, namely, the formation 
and union of certain mesenchymal spaces. 
The following paper was completed August 28th, 1912, and a 
considerable of the later work on it was done at the Institute 
of Anatomy of the University of Minnesota. 
IX. EXPLAXATIOX OF PLATES 19-21, 
Illustrating Mr. AVilliam F. Alienas paper on ^SStudies on the 
Development of the Veno-Lymphatics in the Tail Region 
of Polistotrema (Bdellostoma) Stouti. First 
Communication : Formation of the Caudal Hearts.’^ 
[AVitli the exception of Figs. 4 and 4 a, which are from dissections of 
an adult and a diagrammatic reconstruction, all figures are from trans- 
verse sections or from accurate graphic reconstructions. 
In the transverse sections all vessels were drawn in outline as tul>es, 
primary mesenchyme as cells with processes or as simple dots in the 
low power drawings. A concentration or centralisation of mesenchyme 
to form an organ is set forth by an increased number of these cells or 
dots. A differentiation of mesenchyme into connective tissue is shown 
by short irregular lines running in all directioiis. Muscle, when seen 
l)y low power, is indicated by fine dots, and cartilage by much coarser 
dots. All of the outlines, including the cells and their protoplasmic 
processes, were made with the aid of a camera lucida. 
Concerning the graphic reconstructions, most accurate drawings 
were made of each, or part of each section, with a magnification of 
100. 200, or 400 diameters. In case of the low-power reconstruction 
drawings, either additional high-power drawings were made, or careful 
comparisons were made with the high power, of each structure drawn. 
At the outset, a base line from which all measurements were taken 
was established by drawing a line at the level of the ventral surface 
