350 
WILLIAM F. ALLEX. 
observations on growing* blood-vessels have been confined 
solely to vessels of tliis character and not to the longitudinnl 
trunks. The above hypothesis would be somewhat comparable 
to the development of the nervous system, where you would 
have the central nervous system being formed from one 
primary layer of the embryo, and the nerves, wliich would be 
analogous to the intersegmental vessels, arising as offshoots 
from the main stem. 
The rate of development of the caudal heart is rather slow : 
it begins in embryos between 15 and 20 mm. and is taking- 
place in embryos of 85 mm. This construction stage doubtless 
occupies a period of several months. 
The caudal heart of Polls to trem a differs from the lymph- 
sacs of birds and mammals in that it never loses its primary 
connection with the vein and accpiires a second communication. 
In the adult it is simply an expansion of the vein. 
Concerning the construction of the caudal hearts of Polis- 
totrema, they have been described as having been excavated 
out of the mesenchyme dorsad and ventrad of the anterior 
ends of the two forks of the caudal vein, through the disinte- 
gration of the walls of large mesenchymal cavities, which for 
the most part are connected with each other and the vein, 
and which were originally formed from the isolated mesen- 
chymal spaces after the manner set forth on p. 338. The 
process involved in the formation of these isolated mesenchymal 
spaces is identical to what Lancaster found in the leach, namely, 
a vacuolation of the mesenchyme by the disintegration of 
certain of the mesenchymal cell processes, and the flattening 
of some mesenchymal cells to become endothelium and a 
rounding of others to become red corpuscles. 
In connection with the 20 mm. series A, the possibility was 
noted of the proximal ends of three so-called dorsal embryonic 
intersegmental veins or capillaries contributing in a very 
limited extent to the formation of the caudal hearts. 
It will be seen, then, that the development of the caudal 
hearts in Polls totrema as I have interpreted them is in 
harmony with what Stromsten found in the turtle, but at 
