62 ARACHN1DA. 



As far as we can gather from the statements of Balfour, Schimkewitsch, 

 Loot, and Morix, the fusion of the ganglia of the cheliceral segment with the 

 brain seems to take place very early, and we may perhaps, therefore, assume 

 that their transverse commissure is from the first formed in front of the 

 oesophagus. This commissure has been described by Balfour. 



The Araneae resemble the Scorpiones with regard to the fusion of the cheliceral 

 eanelia with the brain, and we would here call attention to what is perhaps a 

 corresponding process in the Crustacea and in Peripatus (fusion of the ganglia 

 of the second antennae or maxillae with the brain, Vol. ii., pp. 163, 165, and 

 Vol. iii., p. 193). According to Schimkewitsch, there is another pair of ganglia 

 between the cheliceral ganglia and the brain, which corresponds to that part of 

 the brain which gives origin to the (unpaired) rostral nerve. To this point we 

 shall return later. 



After the ganglia have assumed the ahove-mentioned position, they 

 become closely surrounded by a thin layer of mesoderm, which 

 further grows in between the cell-masses and between the fibrous 

 strands. This gives origin to the neurilemma and the trabecular net- 

 work between the separate parts of the ganglia (Schimkewitsch). 

 This is also said to take place in the Crustacea (Vol. ii., p. 162). 



The supra-oesophageal ganglion originates as a large thickening 

 of the cephalic lobes. According to Kishinouyb (No. 62) the 

 rudiment of the brain is continuous with those of the ganglionic 

 chain, these rudiments forming together two longitudinal bands 

 running from before backward. There are at first two distinct 

 thickenings of the cephalic lobes, one on each side, bounded 

 anteriorly by the semi-lunar furrow already described (Fig. 28 B). 

 These furrows become deep and narrow slits, which appear to be 

 lined with an epithelium, and remain in close connection with the 

 rudiment of the brain. When this latter becomes detached from 

 the surface layer, the invaginated ectodermal parts also become cut 

 off from this layer as closed vesicles, and form an integral part of 

 the supra-oesophageal ganglion (Salensky, Balfour, Morin). The 

 invaginate lining of the semi-lunar depressions (cephalic pits) yields 

 the upper hemispherical lobes of the brain; the cavities being 

 retained for some time at the sides. Between the two central 

 masses fibres form, giving rise to the commissures that connect the 

 two halves of the brain. Balfour further distinguishes a ventral 

 region lying immediately in front of the cheliceral ganglia, which 

 perhaps corresponds to the rostral ganglia described by Schimkewitsch. 



The manner in which the brain forms in the Araneae has not yet been ascer- 

 tained with sufficient certainty ; but various accounts given tend to confirm the 

 view that the brain arises here, as in the Scorpiones, as a paired thickening of 

 the ectoderm in connection with paired ectodermal invaginations. These 





