TAlL-liEGrON OF 1‘OEISTOTF E:\IA (hUE[.LOS'1'OMa) STOUTl. 347 
•closer examimitioii of the in. cordis caudalis will reveal it to 
be still in a very embryonic condition. Syncytii have formed 
of the myoblasts, and a few fibrilhn (Fig. 23, Fth.) are 
present ; the general state of the muscle is about identical 
4o tlie myotonies of the 20 mm. series. On the other hand 
the myotonies of the 60 mm. series have developed well- 
formed muscle-fibres (Fig. 23, Mijo. F.). 
In the 85 mm. series the first trace of muscle-fibres (Fig. 24, 
M.C.F.F.) have appeared in the m. cordis caudalis. They are 
still very small, and must increase considerably in diameter 
before adult conditions are reached. It may be said that in 
-every way the m. cordis caudalis muscle-fibres resemble 
skeletal muscle rather than cardiac muscle. Turning to the 
myotonies for comparison we find that the muscle-fibres 
■(Fig. 24, Myo. F.) have about reached adult conditions as 
regards size, but their fibrillm (Fih.) are large and still few 
in numbers, and must subdivide considerably before an adult 
state is reached. 
The m. cordis caudalis might be said then to have its 
origin in a very early stage (20 mm.) from myoblasts situated 
in a condensed mass of mesenchyme that is located between 
the base of the embryonic myotonies on one side and the 
embryonic caudal heart and the median ventral cartilaginous 
bar on the other side. For the most part this mass of cells 
is migrating mesenchyme that is moving mesad to form the 
connective-tissue layer for the caudal heart and the median 
ventral cartilaginous bar, but from our knowledge of the 
histogenesis of skeletal muscle, which this is, and the fact 
that it is innervated by the same ventral spinal nerve that 
innervates the myotonies, and that this nerve reaches the 
region of the m. cordis caudalis before the myoblasts are 
■differentiated from the adjacent mesenchyme, leads one to 
believe that certain muscle plate cells must also have 
migrated inward along with the mesenchyme, and existed 
for some time in the above-described concentrated mesen- 
chyme, and while they are indistinguishable from the sur- 
irounding mesenchymal cells, yet they would have retained 
