348 
WILLIAM F. ALLEX. 
their identity, and eventually will develop into m. cordis 
myoblasts. The first appearance of the in. cordis candalis 
occurs much later than the myotonies, and its fibres never 
reach anything like the size of the myotome fibres. Figs. 21 
to 25 are intended to show the comparative stages of histo- 
genesis of these two muscles from 20 mm. to 20 cm. 
YIII. Summary and General Discussion. 
Polistotrema in the adult possesses a distinct system of 
lymphatics or veno-lymphatics, supphfing a region amply 
furnished with veins. Ko capillaries were observed connect- 
ing these vessels with the arteries, but they always contained 
some red corpuscles, usually great quantities in embryos, and 
only a few in the adult. In all of the larger and medium- 
sized embryos the connective tissue outside these vessels was 
so filled with red corpuscles as to resemble germinative 
centres, and the vessels themselves appeared to be reservoirs 
for storing them. On account of the loose state of the 
endothelium of these vessels it would be an easy matter for 
an external corpuscle to migrate through the wall, and in 
this instance it would seem to be a more plausible expla- 
nation for the appearance of red corpuscles in the veno- 
lymphatics than to regard them as extravasations from the 
blood-vessels caused by the rupturing of a delicate wall sepa- 
rating them. All of the subcutaneous canals are decidedly 
sinus-like, distinctly recalling the conditions found in the 
Amphibians. Posteriorly the entire system culminates in 
two pulsating hearts, which are mere enlargements of the 
two terminal forks of the caudal vein. 
8o far as is known, Polistotrema, Myxine, and the 
eels are the only fish-like vertebrates that possess pulsating 
caudal hearts. They swum by a snake-like movement, while 
most fish swim by rapid lateral vibrations of the caudal fin, 
and would, therefore, not require any specialized pulsating 
heart, for the motion of the fin against the wall of water 
would alternately press these caudal sinuses against the 
