NOTES ON THE DEVELOPMENT OF THE ARANEINA. 101 



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 epiblast. 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 ah.g.). In 

 fig. 20, in which the abdomen is cut horizontally and longi- 

 tudinally, there are seen the two abdominal cords {ab. g.) 

 united by two transverse 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, 

 but meet, in the usual way, in the ventral line. Trans- 

 verse commissures have become established (fig. 20 c,pd. g.), 

 between 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 Lepidoptera, and he states that it gives rise 

 to the transverse commissures between the ganglia. My ob- 

 servations show that for the spider, at any rate, nothing of 

 the kind is present. 



As shown in the longitudinal section (PL X, fig. 21), 

 the ganglion of the chelicerse has now united with the supra- 

 oesophageal ganglion. It forms, as is shown in fig. 20 Z> {ch. g.), 

 a part of the oesophageal commissure, and there is no sub- 

 oesophageal commissure uniting the ganglia of the chelicerse, 

 but the oesophageal ring is completed below by the ganglia 

 of the pedipalpi (fig. 20 c,pd. g.). 



The supra-oesophageal ganglia have become completely 

 separated from the e])iblast. 



I have unfortunately not studied their constitution in the 

 adult, so that I cannot satisfactorily identify the parts which 

 can be made out at this stage. 



I distinguish, however, the following regions : 



(1) A central region containing the commissural part, 

 and continuous below with the ganglia of the chelicerse. 



(2) A dorsal region formed of two hemispherical lobes. 



