NERVOUS SYSTEM. 



339 



from two cords which eventually coalesce '. It is not certain how far the 

 supraoesophageal ganglia are usually in the first instance continuous 



si 



\ 



y 





Fig. 24:i. Section through the pbocephalic lobes of an embryo of 

 Agelena labyrinthica. 

 St. stomadseum ; gr. section through semi-circular groove in procephalic lobe ; 

 ce.s. cephalic section of body cavity. 



with the ventral cord. In Astacus, the early stages of which have 

 been elaborately investigated by Reichenbach (No. 331), they are 

 stated to be so ; the supraoesophageal ganglia are moreover de- 

 scribed by this author as having a somewhat complicated origin. 

 Five elements enter into their composition. There is first formed 

 a pair of pits on the procephalic lobes, which become very deep 



nie.s 



Fig. 245. Two transverse sections through the embryo of Hydrophilus. 

 (After Kowalevsky.) 



A. Transverse section through an embryo in the region of one of the stigmata. 



B. Transverse section through an older embryo. 



vn. ventral nerve-cord ; am. amnion and serous membrane ; me. mesoblast; me.s. 

 somatic mesoblast; hy. hypoblast (?) ; yk. yolk-cells (true hypoblast) ; st. stigma of 

 trachea. 



^ Eeichenbach (No. 331) holds that the walls of the groove between the two strands 

 of the ventral cords become invaginated and assist in the formation of the ventral cord. 



22—2" 



