TAIL-KEGIUN OF POLISTOTIIF MA (BDE LLOSTOMA) STOLI'l’I. 353 
of the iiotocliorcl at riglit angles to a iiieclian sagittal plane, and 
this l)ase-line was added to each successive drawing by accurately 
placing the second or following drawing over the first al>ove a plate 
of glass illnminated from below. In every reconstruction this l>ase 
line is indicated by the dotted line {B. L.), and from it all tlie vessels 
and organs were measured oft with dividers and plotted on ruled 
mm. papei-. 
The caudal artery in all the reconstructions is represented by circular 
cross-lines, the spinal cord by oblique cross-lines, the caudal vein and 
caudal heart by outlines only ; the notochord, when completely recon- 
structed, by coarse dots, otherwise simply as a line to indicate its lower 
border. Certain mesenchymal spaces that are taking part in the for- 
mation of the caudal heart are indicated in outline. Of these there are 
two kinds : those indicated l>y small (c.) have not at this stage attained 
connection with the caudal vein ; and another class, indicated l)y 
capital (C.), that have gained one or more connections with the caudal 
vein. When these cavities are located above or l)elow or in front 
(laterad) of the caudal vein or other organ they are indicated in outline, 
but when they lie median to them they are indicated in dotted out- 
line. Also any vessel lying median to an organ or other vessel is shown 
in dotted lines. A layer of mesenchyme separating the caudal vein 
from a mesenchymal space or two caudal heart mesenchymal spaces, or 
the caudal heart from the ventral veno-lymphatic trunk, is represented 
by a solid black space.] 
I 
List of Abbreviations used in the Figures. 
A. or F. prefixed to an abbreviation signifies anterior or posterior, 
and B. or L. right or ieft. Embryonic structures are indicated by 
small letters, and adult or nearly adult by capital letters. 
a. In reconstructions, marks the place of bifurcation of the caudal 
artery. A. L. Hsi. T. u)- Anterior fork of the posterior longitudinal 
hsemal lymphatic trunk, h. In reconstructions, marks the place of 
union of the two caudal veins. B. L. In reconstructions, base line from 
which all measurements were made. B. V. Blood-vessel. C. Caudal 
heart mesenchymal space that had assumed connection with the caudal 
vein or with other caudal heart mesenchymal spaces, c. Primitive 
caudal heart mesenchymal space that had not at this stage assumed 
connection with the caudal vein or with other caudal heart mesen- 
chymal spaces. Cau. A. Caudal artery. Cau. V. Caudal vein. C. T. 
Connective tissue, c. t. Embryonic connective tissue, d. In Fig. 4 a, 
large veno-lymphatic sinus lying on the lateral surface of the last two 
mucous sacs. D. A. Dorsal fin-ray artery. D. Bar. Median dorsal 
