OLIGONEPHRIDIA 



255 



stom 



at the cephalic end of the embryo occurred and where the envelopes 

 rupture. Shrinkage of the envelopes and a revolution of the embryo 

 then take place (Fig. 178), the dorsal (ental) membrane becoming thick 

 with closely approximated cells (entm), the amnion and serosa enclosing 

 the yolk. When, with the shortening of the embryo, the revolution is 

 completed, the serosa has become greatly contracted in the neck region 

 to form the secondary dorsal organ (Fig. 179, do) which later is absorbed 

 with the yolk, the amnion forming 

 a provisional dorsal closure. 

 Subsequently the dorsal wall of 

 the embryo is completed by 

 the dorsad-growing ectoderm, the 

 amnion being absorbed in the yolk. 

 An embryonic ventral neck gland 

 is developed in Pediculus as de- 

 scribed for some other insects also. 

 The mycetom, which before re- 

 volution is located at the anterior 

 end of the egg, after revolution is 

 carried posteriorly to a position in 

 the yolk in the region of the future 

 mid-gut (Fig. 179, myct). As soon 

 as the anlage of the mid-gut is 

 formed, the mycetom is pushed 

 into the still irregularly formed 

 mid-gut epithelium, developing 

 there a pocket, or diverticulum. 

 The diverticulum at first encloses 

 some yolk in addition to the 

 mycetom. Its connection with 

 the mid-gut becomes so constricted 

 that only a narrow canal unites them, through which the syncitium com- 

 posing the mycetom and the yolk is discharged, leaving the symbionts 

 behind in the pocket. The canal then closes, the syncitium later degener- 

 ating in the yolk. The diverticulum, or pocket, containing the symbionts, 

 enveloped by a layer of mesoderm cells, may now be designated as the 

 "second mycetom" and forms the structure that in the larval louse has 

 been called the "stomach disk." 



The development of the nervous system, as described by Scholzel 

 (1937), offers nothing especially characteristic. Tracheal invaginations 

 were observed by him on the margins of abdominal segments three to 

 eight and dorsally on the metathorax, but none was observed on the 

 mesothorax. The stomodaeum becomes greatly elongated as develop- 



proct 



Fig. 179. — Pediculus. Sagittal section of 

 embryo after revolution, (awi) Amnion, {do) 

 Secondary dorsal organ, {entm) Ental mem- 

 brane, {myct) Mycetom. {nc) Nerve cord. 

 {prod) Proctodaeum. {stom) Stomodaeum. 



