TAIL-KEGION OF rOLlSTOl’FFMA (HDFLLOSTOM A) STOUTL 351 
variance with a considerable recent work on the development 
of the lymph-sacs in the higher vertebrata. 
Closely associated with the early construction of the caudal 
hearts occurs the formation of the ventral trunk, at first 
paired, and each consisting* of a longitudinal chain of isolated 
mesenchymal cavities, which later become connected with 
each other and the caudal heart. 
The paired valves guarding the orifices of the anterior and 
posterior communications between the ventral veno-lymphatic 
trunk and the caudal heart take their origin from a migration 
inward and dorsad of the mesenchymal wall on either side of 
the orifice. 
The endothelium has already been described as coming 
from the flattening of some of the border-cells of the larger 
mesenchymal cavities, and in like manner the connective- 
tissue endocardium comes from a concentration and difieren- 
'tiation of the mesenchyme about the endothelial lining. 
There is no true myocardium in the caudal heart of 
Polist otrema, but the functional myocardium, the M. cordis 
caudalis, arises from myoblasts in the centre of a mass of 
concentrated mesenchyme situated between the base of the 
myotomes and the caudal heart that is migrating inward to 
form the median ventral cartilaginous bar and the connective- 
tissue layer of the caudal heart. Since the muscle-fibres of 
the M. cordis caudalis are true skeletal muscle, their myoblasts 
are supposed to have originated from primitive muscle plate 
cells that have migrated inward with the surrounding mesen- 
chyme. It was demonstrated that the ventral spinal nerve 
which innervated this muscle also supplied the myotomes, and 
in the early embryos reached the area of the future M. cordis 
caudalis long before its myoblasts were distinguishable from 
the adjacent mesenchyme. 
In conclusion it might be said that my studies thus far 
indicate that the most primitive form of a lymphatic system 
are veins that function for both lymphatics and veins. Hence 
it would be expected that ontogeny would repeat the 
phylogeny of the lymphatics, and instead of having their 
