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WILLIAM F. ALLEX. 
85 mm. series was reached. Here in the extreme posterior 
ventral corner, a ventral intersegmental vein pierced the im 
cordis caudalis and terminated in the left candal heart through 
a very small orifice. This vein I took to be the homologne of 
a more anterior vein in the 20 cm. series, which emptied into 
the anterior end of the heart at about its median line. This 
would indicate, then, that the permanent intersegmental veins 
emptying into the candal hearts are of late formation, 
beginning somewhere in embryos of between 60 and 85 mm. 
in length. 
As regards the intersegmental arteries in the caudal heart 
region, many of them in the 20 mm. series ]^, 25 mm., 27 mm., 
and 60 mm. series have retained their primitive condition, as 
in 20 mm. series xA., of having dorsal and ventral intersegmental 
arteries, arising separately from the caudal arteries; while in 
some instances, as in the 27 mm. embryo, the basal parts of 
these arteries have united in a common stem (Fig. 10, hits. H.). 
This part of the process of the development of the inter- 
segmental arteries of Polistotrema may be, then, somewhat 
analogous to the spinal ganglion cells in the mammalian 
embryo changing from bipolar to unipolar as development 
advances. 
IV. Development oe the Caudal Hearts in Polistotrema. 
In considering the mode of formation of the candal hearts 
in Polistotrema the condition of the candal hearts in each 
of the various stages will be discussed, beginning with the 
youngest. Of these there are two well-preserved embryos of 
20 mm. in length, which have been designated as series A and 
B; although of equal length, practically all of the structures 
are considerably more advanced in series B. 
20 MM. Polistotrema Series. 
In both series and B, the caudal hearts are shown by 
lateral I’econstructions of the left embryonic caudal heart 
(Figs. 6 and 7) and by several transverse sections (Figs. 11 to 
