NOTES ON THE DEVELOPMENT OF THE ARANEINA. 175 
differentiated. The caudal lobe, though less broad than the 
procephalic lobe, is still a widish structure. The most 
important internal changes concern the mesoblast, which is 
now imperfectly though distinctly divided into somites, 
corresponding with segments visible externally. Each meso- 
blastic somite is formed of a distinct somatic layer closely 
attached to the epiblast, and a thinner and less well-marked 
splanchnic layer. In the appendage-bearing segments the 
somatic layer is continued up into the appendages. 
The epiblast is distinctly thinner in the median line than 
at the two sides. 
The next stage figured (PL XIX, figs. 5 and 6) is an im- 
portant one, as it is characterised by the establishment of 
the full number of appendages. The whole length of the 
ventral plate has greatly increased, so that it embraces 
nearly the circumference of the ovum, and there is left 
uncovered but a very small arc between the two extremities 
of the plate (PI. XIX, fig. 6 ; PI. XX, fig. 15). This arc 
is the future dorsal portion of the embryo, which lags in its 
development immensely behind the ventral portion. 
There is a very distinctly bilobed procephalic region {pr, 1) 
well separated from the segment with the cheliceree {ch). It 
is marked by a shallow groove opening behind into a circular 
depression (^L) — the earliest rudiment of the stomodseum. 
The six segments behind the procephalic lobes are the six 
largest, and each of them bears two prominent appendages. 
They constitute the six appendage-bearing segments of the 
adult. The four future ambulatory appendages are equal in 
size : they are slightly larger than the pedipalpi, and these 
again than the chelicerm. Behind the six somites with pro- 
minent appendages there are four well-marked somites, each 
with a small protuberance. These four protuberances are 
provisional appendages. They have been found in many 
other genera of Araneina (Claparede, Barrois). The segments 
behind these are rudimentary and difficult to count, but 
there are, at any rate, five, and at a slightly later stage 
probably six, including the anal lobe. These fresh segments 
have been formed by the continued segmentation of the anal 
lobe, which has greatly altered its shape in the process. The 
ventral groove of the earlier stage is still continued along 
the whole length of the ventral plate. 
By the close of this stage the full number of post-cephalic 
segments has become established. They are best seen in the 
longitudinal section (PI. XX, fig. 15). There are six anterior 
appendage-bearing segments, followed by four with rudimen- 
tary appendages (not seen in this figure), and six without 
