TAIL-REGIOX OB’ POLISTOTKEMA (bDELEOSTOMa) STOUTI. 325 
anastomosed with a corresponding ventral intersegmental 
artery, and several such coinnuinications were observed further 
caiulad. No embryonic ventral intersegmental veins were 
found in the caudal region. From the lateral reconstruction 
(Fig*. 7) several of the larger mesenchymal spaces (C.) might 
readily be taken for embryonic dorsal intersegmental veins, 
but transverse sections demonstrate their position and course 
to be too far mesad to be considered as such. 
In the 20 mm. series B, 25 mm., 27 mm., and 60 mm. series 
there are no embryonic intersegmental veins in the region of 
the caudal heart, nor is there any evidence in these series of 
a degeneration of the embryonic veins shown in the 20 mm. 
series A. Also there is absolutely no reason for supposing 
that the degenerating endothelium of these intersegmental 
veins formed the cavities {0. and c.), in the reconstructions 6 
to 10, for not only is their position and irregular arrange- 
ment against it, being located too far laterad, but instead of 
these spaces in the mesenchyme being lined with flattened 
endothelium they are bordered by undifferentiated mesen- 
chyme. In the 20 mm. series B, 25 mm. and 27 mm. series 
these intersegmental veins have survived behind the caudal 
heart and are numerous. Posterior of the caudal heart in the 
20 mm. series B, the embryonic intersegmental veins resemble 
the embryonic intersegmental veins of the heart region in the 
more embryonic 20 mm. series A, and in several places tho 
veins were seen to anastomose with the arteries. Xowhere in 
the caudal region of the 20 mm. series B were any embryonic 
intersegmental veins given off ventrad; though in the 27 mm. 
series they were in the process of formation, apparently as 
sprouts from the caudal vein. In all of the embryos these 
embryonic intersegmental veins had at first a lateral course, 
and then bent dorsad or ventrad, being situated some little 
distance from the notochord, for the most part too laterad, to 
take any part in the formation of the so-called caudal heart 
mesenchymal spaces (C. and c.). Xo trace of well-formed 
intersegmental veins emptying directly into the heart, as have 
already been described for the adult, vrere seen until the 
VOL. 59, PART 2. NEW SERIES. 21 
