NOTES ON THE DEVELOPMENT OF THE ARANEINA. 99 



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 seems to 

 me in the highest degree improbable, two of them remain 

 as the spinning mammillae, 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. VIII, tig. 9. Dor- 

 sally the heart is now very conspicuous, and in front of the 

 chelicerae may be seen the supra-cesophageal 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. X, 

 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. X, fig. 22 s). The septa so formed, which 

 were first described by Barrois, are not complete. The septa 

 of the two 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 

 they play an important part in the subsequent development 



