338 
willia:\[ f. allex. 
genetic study of a veiio-lympliatic heart. A striking diffe- 
rence between the caudal heart of Polistotrema and the 
embryonic sacs of the higher vertebrata is that the caudal 
hearts of Polistotrema^ when once formed, never lose 
their primary connection with the vein, and acquire a second 
communication. It was shown in the adult to be a con- 
spicuous swelling of the two ])Osterior branches of the caudal, 
and to be formed after the manner outlined below. 
From the previous description it will be seen that the main 
process involved in construction of the caudal hearts in 
Polistotrema consists, first in the formation of certain 
isolated mesenchymal spaces in the region of the anterior 
ends of the two branches of the caudal vein, by the breaking 
down of certain mesenchymal cell processes in the centre, 
and the thickening of others to form the boundaries of the 
cavity. Some of the cells in the centre may become spherical, 
increase in size, and eventually become transformed into red 
corpuscles. The next stage results in the breaking-down of 
the mesenchymal partition between this cavity and the 
candal vein. About this time other more distal isolated 
mesenchymal spaces will occur. These will increase in size, 
meet and unite with the above-mentioned connected mesen- 
chymal cavity, and frequently the distal ends of these cavities 
will coalesce in a larger cavity that will come in contact with, 
and join, the caudal vein by the disintegration of the partition 
separating them, thus establishing a second connection vvith 
the vein for above-mentioned cavities. While this is being- 
accomplished, or shortly after, the nmsenchymal walls 
separating the middle spaces will become broken down, 
leaving one lai-ge mesenchymal space that is connected at 
either end with the caudal vein. Apparently, in places the 
border mesenchymal cells of this cavity are flattening to 
contribute to the endothelium of the heart, while certain of 
the enclosed cells, as was noted for the isolated spaces, had, 
by increasing in size and becoming spherical, differentiated 
into red corpuscles. In like manner the mesenchyme on all 
sides of the anterior portion of the two forks of the caudal 
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