NOTES ON THE DEVELOPMENT OF THE ARANEINA. 181 
The dorsal region has, however, not only increased in length, 
but also in definiteness, and a series of transverse markings 
(fig. 8 a and which are very conspicuous in the case of 
the four anterior abdominal segments (the segments with 
rudimentary appendages), have appeared, indicating the 
limits of segments dorsally. The terga of the somites may, 
in fact, be said to have become formed. The posterior terga 
(fig. 8 d) are very narrow compared to the anterior. 
The caudal protuberance is more prominent than it was, 
and somewhat bilobed ; it is continued on each side into 
one of the bands, into which the ventral plate is divided. 
These bands, as is best seen in side view (fig. 8 h), 
have a ventral curvature, or, perhaps more correctly, 'are 
formed of two parts, which meet at a large angle open to- 
wards the ventral surface. The posterior of these parts bears 
the four still very conspicuous provisional appendages, and 
the anterior the six pairs of thoracic appendages. The four 
ambulatory appendages are now seven-jointed, as in the 
adult, but though longer than in the previous stage they 
do not any longer cross or even meet in the middle line, but 
are, on the contrary, separated by a very considerable in- 
terval. This is due to the great distension by the yolk of 
the ventral part of the body, in the interval between the 
two parts of the original ventral plate. The amount of this 
yolk may be gathered from the section (PI. XXI, fig. 18). 
The pedipalpi carry a blade on their basal joint. The che- 
licerse no longer appear to spring from an independent post- 
oral segment. 
There is a conspicuous lower lip, but the upper is less 
prominent than before. Sections at this stage show that 
the internal changes have been nearly as considerable as the 
external. 
The dorsal region is now formed of a (1) flattened layer of 
epiblast cells, and a (2) fairly thick layer of large and rather 
characteristic cells which any one who has studied sections 
of spider’s embryos will recognise as derivatives of the yolk. 
These cells are not, therefore, derived from prolongations 
of the somatic and splanchnic layers of the already formed' 
somites, but are new formations derived from the yolk. They 
commenced to be formed at a much earlier period, and some 
of them are shown in the long section (PI. XX, fig. 15). 
In the next stage these cells become differentiated into the 
somatic and splanchnic mesoblast layers of the dorsal region 
of the embryo. 
In the dorsal region of the abdomen the heart has already 
become established. So far as I have been able to make 
