448 



ARACHNIDA. 



segment of the chelicerae, lie immediately behind the stomodsum, 

 and are as independent of each other as the remaining ganglia. 

 Anteriorly they border on the supra-oesophageal ganglia. When 

 the yolk sack is formed in connection with the ventral flexure of 

 the embryo, the two nerve-cords become very widely separated 

 (fig. 206, vn) in their middle region. At a later period, at the 

 stage represented in fig. 201 B, they again become approximated 

 in the ventral line, and delicate commissures are formed uniting 



FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELENA 

 LABYRINTHICA AT THREE STAGES. 



A. Stage when about three segments are formed. The mesoblastic plate is not 

 divided into two bands. 



B. Stage when six segments are present (fig. 200 B). The mesoblast is now 

 divided into two bands. 



C. Stage represented in fig. 200 D. The ventral cords have begun to be formed 

 on thickenings of the epiblast, and the limbs are established. 



ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; vn. ventral nerve-cord ; 

 yk. yolk. 



the ganglia of the two sides, but there is no trace at this or any 

 other period of a median invagination of epiblast between the 

 two cords, such as Hatschek and other observers have attempted 

 to establish for various Arthropoda and Chaetopoda. At the 

 stage represented in fig. 20 1 A the nerve ganglia are still present 

 in the abdomen, though only about four ganglia can be distin- 

 guished. At a later stage these ganglia fuse into two continuous 



