174 
F. M. BALFOUR. 
ventral thickening of the embryo now occupies rather more 
than half the circumference of the ovum. 
Transverse sections show that considerable changes have 
been effected in the constitution of the blastoderm. In the 
previous stage, the ventral plate was formed of an uniform 
external layer of epiblast, and a continuous internal layer 
of mesoblast. The mesoblast has now become divided along 
the whole length of the embryo, except, perhaps, the proce- 
phalic lobes, into two lateral bands which are not continuous 
across the middle line (PI. XX, fig. 13 me). It has, more- 
over, become a much more definite layer, closely attached 
to the epiblast. Between each mesoblastic band and the 
adjoining yolk there are placed a few scattered cells, which 
in a somewhat later stage become the splanchnic mesoblast. 
These cells are derived from the yolk-cells ; and almost 
every section contains examples of such cells in the act of 
joining the mesoblast.. 
The epiblast of the ventral plate has not, to any great 
extent, altered in constitution. It is, perhaps, a shade 
thinner in the median line than it is laterally. The division 
of the mesoblast plate into two bands, together, perhaps, 
with the slight reduction of the epiblast in the median 
ventral line, gives rise at this stage to an imperfectly 
marked median groove. 
The dorsal epiblast is still formed of a single layer of flat 
cells. In the neighbourhood of this layer the yolk nuclei 
are especially concentrated. The yolk itself remains as 
before. 
The segments continue to increase regularly, each fresh 
segment being added in the usual way between the last 
formed segment and the unsegmented caudal lobe. At the 
stage when about nine or ten segments have become estab- 
lished, the first rudiments of appendages become visible. 
At this period (PI. XIX, fig. 4) there is a distinct median 
ventral groove, extending through the whole length of the 
embryo, which becomes, however, considerably shallower 
behind. The procephalic region is distinctly bilobed. The 
first segment (that of the chelicerge) is better marked off 
. from it than in the previous stage, but is without a trace 
of an appendage, and exhibits therefore, in respect to the 
development of its appendages, the same retardation that 
characterised its first appearance. The next five segments, 
viz, those of the pedipalpi and four ambulatory appendages, 
present a very well-marked swelling at each extremity. 
These swellings are the earliest traces of the appendages. 
Of the three succeeding segments; only the first is well 
