NG. 2°) LIMB-MUSCLES [IN AMPHIBIA. 113 
rise only to muscle tissue. They are to be regarded rather 
as the ‘“ formative-tissue’’! cells of Goette and Ziegler. In 
the elasmobranchs, where ‘‘ muscle-buds”’ are best developed, 
Ziegler has shown that.cell-proliferation occurs independently 
of the formation of muscle-buds and in addition to it. 
The first indication of the posterior limbs is seen in a thick- 
ening of the somatopleure at the extreme posterior limit of the 
coelom. The limbs arise ventral to the myotomes and appear 
first as an aggregation of a few mesenchyme cells lying directly 
beneath the thickened ectoderm (Pl. X, Fig. 13). By the time 
the posterior limbs first begin to make their appearance the 
yolk-granules have been almost wholly absorbed from the 
mesoderm, leaving the large nuclei lying freely suspended at 
the nodal points of a protoplasmic network. In this network, 
which is almost colorless even in stained sections, the nuclear 
divisions can be easily followed. Since cell migration is of 
widespread occurrence in the formation of embryonic organs, 
evidence based upon nuclear division alone cannot be re- 
garded as entirely conclusive in determining the original 
sources of the cells that go to form any given tissue. Never- 
theless, whatever evidence can be deduced from karyokinesis 
points unmistakably to the somatopleure as the region of growth 
in the formation of the posterior limbs. This explanation is 
rendered all the more probable by the frequent proliferation of 
cells from the endothelium bordering the coelom directly in the 
limb-region. 
Comparing the total number of cases of mitosis at the ven- 
tral edges of the myotomes in the limb-region of eleven embryos 
with the total number of cases in the limb-rudiment, we find 
that there are 24 dividing nuclei at the edges of the myotomes 
to 242 in the somatopleure of the limb; or 1 in the myotome 
to every 10 in the somatopleure. 
When we consider that numerous cases of mitosis are also 
seen at the ventral edges of the myotomes throughout the 
length of the body and in the tail, the fact that isolated cases 
of cell division occur at the edges of the myotomes in the limb- 
1 I have used “ formative tissue ” in Ziegler’s sense as referring to cells that 
are physiologically undifferentiated. 
