26o THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



FIG. 44. Three diagrams representing the manner in which the head discs and 

 nervous system of the embryo are related / and 2, before the retraction, and 

 j, after the retraction, of the fore-head to form the cephalo-pharyngeal sac : 

 c, the crop ; g, cephalic ganglion ; i 1 , antennal, and i~, optic discs ; nix, first pair 

 of maxillae ; /A, cephalo-pharynx ; r, the ring ; s, salivary duct ; sf, stomodreum ; 

 v, hollow ventricle of the brain ; v g, ventral ganglia. The invaginated epiblast 

 in j is represented by a dark single line in / and 2. The dotted lines in 2 and 3 

 trace the course of the tracheal vessels of the discs, which arise close to the position 

 of the future anterior spiracle. / and 2 represent embryos in the stage from which 

 the sections represented in Plate XV. were taken ; j exhibits the relations of the 

 parts in the young larva. 



