KOTES ON THE DEVELOPMENT OF THE ARANEINA. 183 
requires to be dealt with so far as the external features are 
concerned. The yolk has now mainly passed into the 
abdomen, and the constriction separating the thorax and 
abdomen has began to appear. The yolk sack has become ab- 
sorbed, so that the two halves of the ventral plate in the thorax 
are no longer widely divaricated. The limbs have to a large 
extent acquired their permanent structure, and the rings of 
which they are formed in the earlier stages are now replaced 
by definite joints. A delicate cuticle has become formed, 
which is not figured in my sections. The four rudimen- 
tary appendages have disappeared, unless, which to seems me 
in the highest degree improbable, they remain as the spin- 
ning mammillae, two pairs of which are now present. Be- 
hind is the anal lobe, which is much smaller and less con- 
spicuous than in the previous stage. The spinnerets and 
anal lobe are shown as five papillae in PI. XIX, fig. 9. Dor- 
sally the heart is now very conspicuous, and in front of the 
chelicerae may be seen the supra-oesophageal ganglia. 
The indifferent mesoblast has now to a great extent 
become converted into the permanent tissues. On the dorsal 
surface there was present in the last stage a great mass of 
unformed mesoblast cells. This mass of cells has now 
become divided into a somatic and splanchnic layer (PI. XXI, 
fig. 22). It has, moreover, in the abdominal region at any 
rate, become divided up into somites. At the junction be- 
tween the successive somites the splanchnic mesoblast on 
each side of the abdomen dips down into the yolk and forms 
a septum (PI. XXI, fig. 22 s). The septa so formed, which 
were first described by Barrois, are not complete. The septa 
of the t^vo sides do not, in the first place, quite meet along 
the median dorsal or ventral lines, and in the second place 
they only penetrate the yolk for a certain distance. In- 
ternally they usually end in a thickened border. 
Along the line of insertion of each of these septa there is 
developed a considerable space between the somatic and 
splanchnic layers of mesoblast. The parts of the body 
cavity so established are transversely directed channels pass- 
ing from the heart outwards. They probably constitute the' 
venous spaces, and perhaps also contain the transverse aortic 
branches. 
In the intervals between these venous spaces the somatic 
and splanchnic layers of mesoblast are in contact with each 
other. 
I have not been able to work out satisfactorily the later 
stages of development of the septa, but I have found that 
tbcy play an important part in tlie subsequent development 
VOL. XX. NEW SEU. 
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