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WILLIAM F. AT.LEX. 
portions and for the ventral portion to receive an interseginental Lrancli. 
X 25. 
Fig. 4. — -Is from a dissection of the left caudal heart and the adjacent 
lymphatic and blood-vessels, excepting the lateral veno-lymphatic sinus 
and its dorsal and ventral or anal fin branches of an adult P o 1 i s t o t r e m a 
are omitted. Any attempt to represent the enormous lateral sinus in 
this drawing would necessarily oliliterate everything that was more 
deep-seated. It should, however, be stated that both the dorsal and 
ventral'canals had frecpient communications with the lateral sinuses. 
X 2. 
Fig. 4 A. — Diagrammatic i-econstruction of the ha3inal lymphatic system 
of a 20 cm. Polistotrema in the region directly cephalad of the caudal 
hearts as seen from the left side. 
Fig. 5. — Transverse section through the tail of a 20 mm. Polisto- 
trema embryo, series B. Section passes through the anterior end of 
the caudal hearts, which are in the process of formation. Primarily it is 
intended to show the position of the embryonic caudal hearts and their 
relation to other developing structures, especially to the emln-yonic 
lateral sinus, which at this stage consists of several mesenchymal spaces 
uniting in front and behind, but which in a later stage will be one con- 
tinuous and much larger cavity. At this stage none of the veno- 
lymphatic trunks have gained connection with each other or with the 
caudal heart. ISlote the large mesenchymal cavity (C.), which in the 
next section caudad (Fig. 13) connects with the caudal vein. Also a 
more ventral cavity (/. v. v.) is shown that has at this stage no connection 
with the caudal vein, and which for reasons stated in the text I take to 
be the beginning of the posterior portion of the ventral veno-lymphatic 
trunk. X 50. 
Fig. 6. — Graphic reconstruction of the left caudal artery, left caudal 
vein, left embryonic caudal heart, and the left embryonic ventral veno- 
lymphatic trunk of a 20 mm. Polistotrema embryo, series B, as seen 
from the left side. Near the tip of the spinal cord a space of 640 
microns has been left out of the reconstruction. Behind this the 
posterior ending of the notochord, spinal cord, and the union of the left 
caudal vein in the left caudal artery is clearly shown. Of especial sig- 
nificance are the mesenchymal spaces, (c.) and (C.), which are undoubtedly 
the main factors in the formation of the caudal hearts. X 200. 
Fig. 7. — Identical reconstimction of the same structures of another 
20 mm. Polistotrema embryo, series A, as Fig. 6, except that this re- 
construction is not carried as far caudad. It should be noted in the 
caudal heart region that there is a direct anastomosis of an interseg- 
mental vein with an interseginental artery. If the ventral veno- 
lymphatic trunk is present it is represented by a small mesenchymal 
