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



Pn;_ AA Three diagrams representing the manner in which the head discs and 



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

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

 c, the crop ; g, cephaHc ganglion ; /', antennal, and P, optic discs ; iiix, 6rst pair 

 of maxillx ; /■/<, cephalo-pharynx ; r, the ring ; s, salivary duct ; st, stomodx'um ; 

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

 in 3 is represented by a dark single line in / and 2. The dotted lines in _' 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 I'late XV. were taken ; j exhibits the relations of the 

 parts in the young larva. 



