TR AC HE AT A. 



formed in connection with the ventral flexure of the embryo, the two 

 A y* 



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. rnesoblastie somite; vn. ventral nerve-cord; 

 t/ft. yolk. 



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 commis- 

 sures are formed uniting 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 Chsetopoda. At 

 the stage represented in fig. 201 A the nerve ganglia are still present in 

 the abdomen, though only about four ganglia can be distinguished. 

 At a later stage these ganglia fuse into two continuous cords, united 

 however by commissures corresponding with the previous ganglia. 



The ganglia of the chelicerae have, by the stage represented in 

 fig. 201 B, completely fused with the supra-cesophageal ganglia and 

 form part of the oesophageal commissure. The cesophageal commis- 

 sure is however completed ventrally by the ganglia of the pedipalpi. 



The supra-cesophageal 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 thickening eventually 

 becomes detached from the superficial epiblast, but before this takes 

 place the two grooves become deeper, and on the separation of the 



24-2 



