TAIL-REGJON OF I’OrilS'l'OTRFMA (dDELLOSTOMA) STOUTI. 345 
fibrillA), and tlie mesenchyme is migrating' inward in places to 
form the internal perimysium {i. per.), while both the dorsal 
and the ventral ends consist only of a mass of nndilferentiated 
myoblasts and mesenchyme, which are continuous with dorso- 
median and ventro-median masses of greatly thickened mesen- 
chyme. In our study the ventro-median mass of concentrated 
mesenchyme (Figs. 5 and 19) are of especial interest. It can 
be traced inward and ventrad to the lateral wall of the caudal 
heart, and below the heart to be continuous with another 
conspicuous concentration of mesenchyme that is situated 
between the two caudal hearts, and is destined to form the 
membranous stage of the median ventral cartilaginous bar 
and the median portion of the connective-tissue wall of the 
caudal heart, Avhile dorsad and ventrad it gradually blends 
in with the adjacent mesenchyme. 
Now the (piestion might be asked, What is the signifi- 
cance of this concentration of mesenchyme, and what will it 
form ? Since cell-division is not particularly active here, it 
is quite certain that this mesenchyme has collected here, 
and is migrating inward to form tlie median ventral cartila- 
ginous bar and the connective-tissue layer of the heart. 
Moreover, from what we know about the manner of forma- 
tion of skeletal muscle in general, and the fact that no sharp 
line of separation can be determined between this mass of 
mesenchyme and the myotonies, together Avith the additional 
fact that the two are innervated by the same nerve, makes 
it reasonable to suppose that Avithin this mass of mesen- 
chyme there are certain primitive muscle plate cells, that 
migrated iuAvard along Avith the mesenchyme, but Avhich, 
although at present indistinguishable from the mesenchymal 
cells, havm preserved their identity, and Avill later multiply, 
arrange themselves outside the heart, and differentiate into 
m. cordis caudalis myoblasts. A point favouring the 
presence of early myoblasts in the m. cordis caudalis region 
is shoAvn by the fact that the third Amntral spinal nerve in 
the heart region, the one Avhich innervates this muscle in the 
adult, has in the embryo (Fig. 11, V. Sp. N.) extended to the 
