686 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



directed involution of the epiblast. I have seen traces of what 

 I believe to be two outgrowths from it, which form the Mal- 

 pighian bodies. 



The next stage constitutes (PL 3.0, fig. 9) the last which 

 requires to be dealt with so far as the external features are con- 

 cerned. The yolk has now mainly passed into the abdomen, 

 and the constriction separating the thorax and abdomen has 

 begun to appear. The yolk-sack has become absorbed, 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 rudimentary appendages have dis- 

 appeared, unless, which seerns to me in the highest degree im- 

 probable, they remain as the spinning mammillae, two pairs of 

 which are now present. Behind is the anal lobe, which is much 

 smaller and less conspicuous than in the previous stage. The 

 spinnerets and anal lobe are shewn as five papillae in PI. 30, fig. 9. 

 Dorsally 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. 32, fig. 22). It has. 

 moreover, in the abdominal region at any rate, become divided 

 up into somites. At the junction between the successive somites 

 the splanchnic mesoblast on each side of the abdomen dips 

 down into the yolk and forms a septum (PI. 32, 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. Internally 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 splanch- 

 nic layers of mesoblast. The parts of the body-cavity so estab- 



