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F. M. BALFOUR. 
appendages behind. There are, therefore, sixteen in all. 
This number accords with the result arrived at by Barrois, 
but is higher by two than that given by Claparede. 
The germinal layers {vide PL XX, fig. 14) have by this 
stage undergone a further development. The mesoblastic 
somites are more fully developed. The general relations of 
these somites is shown in longitudinal section in PI. XX, 
fig. 15, and in transverse section in PL XX, fig. 14. In the 
tail, where they are simplest (shown on the upper side in fig, 
14), each mesoblastic somite is formed of a somatic layer of 
more or less cubical cells attached to the epiblast, and a 
splanchnic layer of flattened cells. Between the two is placed a 
completely circumscribed cavity, which constitutes part of the 
embryonic body cavity. Between the yolk and the splanchnic 
layer are placed a few scattered cells, which form the latest 
derivatives of the yolk-cells, and are to be reckoned as part of 
the splanclinic mesoblast. The mesoblastic somites do not 
extend outwards beyond the edge of the ventral plate, and 
the corresponding mesoblastic somites of the two sides do 
not nearly meet in the middle line. In the limb-bearing 
somites the mesoblast has the same general characters as in 
the posterior somites, but the somatic layer is prolonged as 
a hollow papilliform process into the limb, so that each limb 
has an axial cavity continuous with the section of the body 
cavity of its somite. The description given by Metschnikoff 
of the formation of the mesoblastic somites in the scorpion, 
and their continuation into the limhs, closely corresponds 
with the history of these parts in spiders. In the region of 
each procephalic lobe the mesoblast is present as a continuous 
layer underneath the epiblast, but in the earlier part of the 
stage, at any rate, is not formed of two distinct layers with 
a cavity between them. 
The epihlast at this stage has also undergone important 
changes. Along the median ventral groove it has become 
very thin. On each side of this groove it exhibits in each 
appendage-bearing somite a well-marked thickening, which 
gives in surface views the appearance of a slightly raised 
area (PL XIX, fig. 5), between each appendage and the 
median line. These thickenings are the first rudiments of 
the ventral nerve ganglia. The ventral nerve cord at this 
stage is formed of two ridge-like thickenings of the epiblast, 
widely separated in the median line, each of which is con- 
stituted of a series of raised divisions — the ganglia — united by 
shorter, less prominent divisions (fig. 14, vg). The nerve 
cords are formed from before backwards, and are not at this 
stage found in the hinder segments. There is a distinct 
