688 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



in the thorax (vide fig. 20 c)> There are, in addition to these, 

 intrinsic transverse fibres on the ventral side of the thorax. 

 Besides these muscles there are in the thorax, attached to the 

 suctorial extremity of the stomodaeum, three powerful muscles, 

 which I believe to be derived from the somatic mesoblast. One 

 of these passes vertically down from the dorsal surface, in the 

 septum the commencement of which was described in the last 

 stage. The two other muscles are lateral, one on each side (PI. 

 31, fig. 20 c.). 



The heart has now, in most respects, reached its full de- 

 velopment. It is formed of an outer muscular layer, within 

 which is a doubly-contoured lining, containing nuclei at inter- 

 vals, which is probably of the nature of an epithelioid lining 

 (PI. 32, fig. 22 hf}. In its lumen are numerous blood-corpuscles 

 (not represented in my figure). The heart lies in a space bound 

 below by the splanchnic mesoblast, and to the sides by the 

 somatic mesoblast. This space forms a kind of pericardium 

 (fig. 22 pc\ but dorsally the heart is in contact with the epi- 

 blast. The arterial trunks connected with it are fully established. 



The nervous system has undergone very important changes. 



In the abdominal region the ganglia of each side have fused 

 together into a continuous cord (fig. 21 ab. g.}. In fig. 20, in 

 which the abdomen is cut horizontally and longitudinally, there 

 are seen the two abdominal cords (ab. g^} united by two trans- 

 verse commissures; and I believe that there are at this stage 

 three or four transverse commissures at any rate, which remain 

 as indications of the separate ganglia, from the coalescence of 

 which the abdominal cords are formed. The two abdominal 

 cords are parallel and in close contact. 



In the thoracic region changes of not less importance have 

 taken place. The ganglia are still distinct. The two cords 

 formed of these ganglia are no longer widely separated in 

 median line, but meet, in the usual way, in the ventral line. 

 Transverse commissures have become established (fig. 20 c) be- 

 tween the ganglia of the two sides. There is as little trace at 

 this, as at the previous stages, of an ingrowth of epiblast, to 

 form a median portion of the central nervous system. Such 

 a median structure has been described by Hatschek for Lepi- 

 doptera, and he states that it gives rise to the transverse com- 



