NOTES ON THE DEVELOPMENT OF THE ARANEINA. 185 
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. 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 importa^nt 
changes. 
In the abdominal region the ganglia of each side have 
fused together into a continuous cord (fig. SI ah.g,). In 
fig. SO, 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 
in median line, but meet, in the usual way, in the ventral 
line. Transverse commissures have become established 
(fig. 20 c) 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 XXI, fig. 21), 
the ganglion of the chelicerse has now united with the supra- 
msophageal ganglion. It forms, as is shown in fig. 20 h {ch, g.), 
a part of the oesophageal commissure, and there is no sub- 
oesophageal commissure uniting the ganglia of the chelicerae, 
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 epiblast. 
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 chelicerae. 
(2) A dorsal region formed of two hemispherical lobes. 
