TRACHEATA. 449 



cords, united however by commissures corresponding with the 

 previous ganglia. 



The ganglia of the chelicerae have, by the stage represented 

 in fig. 20 1 B, completely fused with the supra-cesophageal ganglia 

 and form part of the cesophageal commissure. The oesophageal 

 commissure is however completed ventrally by the ganglia of 

 the pedipalpi. 



The supra-oesophageal ganglia are formed independently of 

 the ventral cords as two thickenings of the procephalic lobes (fig. 

 205). The thickenings of the two lobes are independent, and 

 each of them becomes early marked out by a semicircular groove 

 (fig. 200 D) running outwards from the upper lip. Each thick- 

 ening eventually becomes detached from the superficial epiblast, 

 but before this takes place the two grooves become deeper, 

 and on the separation of the ganglia from the epiblast, the 

 cells lining the grooves become involuted and detached from 

 the skin, and form an integral part of the supra-oesophageal 

 ganglia. 



At the stage represented in fig. 201 B the supra-oesophageal ganglia 

 are completely detached from the epiblast, and are constituted of the 

 following parts : (i) A dorsal section formed of two hemispherical lobes, 

 mainly formed of the invaginated lining of the semicircular grooves. The 

 original lumen of the groove is still present on the outer side of these 

 lobes. (2) Two central masses, one for each ganglion, formed of puncti- 

 form tissue, and connected by a transverse commissure. (3) A ventral 

 anterior lobe. (4) The original ganglia of the chelicerce, which form the 

 ventral parts of the ganglia 1 . 



The later stages in the development of the nervous system have not 

 been worked out. 



The development of the nervous system in the Scorpion is almost 

 identical with that in Spiders, but Metschnikoff believes, though without 

 adducing satisfactory evidence, that the median integument between the 

 two nerve cords assists in forming the ventral nerve cord. Grooves are 

 present in the supra-cesophageal ganglia similar to those in Spiders. 



The mesoblast. The history of the mesoblast, up to the 

 formation of a ventral plate subjacent to the thickened plate of 

 epiblast, has been already given. The ventral plate is shewn 

 in fig. 203 A. It is seen to be formed mainly of small cells, 



1 For further details vide self, No. 440. 

 B. II. 29 



