252 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



rapidly passes down the ovarian tubule and finally out by the common 

 oviduct. 



The secondary dorsal organ makes its appearance directly above the 

 stomodaeum after the rupture of amnion and serosa. It is formed by 

 an intucking of the ectoderm (Fig. 174), by which the contents of the 

 organ come in close contact with the wall of the ovarian tubule. Finally, 

 the ectodermal aperture is closed, and the dorsal organ is absorbed along 

 with the nutritive mass. 



Blastokinesis does not take place during development. 



Fig. 174. — Archipsocus. Cross section of secondary dorsal organ, (br) Brain, (ent) 

 Entoderm, (ovt) Ovariole. (stom) Stomodaeum. 



SIPHUNCULATA, THE SUCKING LICE 

 The Head Louse (Pediculus humanus capitis) 



The egg of the head louse is ovoid, somewhat narrowed posteriorly, 

 with an operculum, or cap, at the anterior end containing the micropylar 

 openings. The union of the pronuclei occurs in the anterior third of the 

 egg. Scholzel (1937) found that at the 256-cell cleavage stage the 

 cleavage nuclei are uniformly distributed over the yolk, placed at an 

 average distance of 10 to 15 microns from the periplasm. When the 

 nuclei reach the periphery, they continue dividing mitotically, forming 

 a complete blastoderm. The vitellophags which remain behind in the 

 yolk in the main have smaller nuclei and do not undergo mitotic division. 

 On the posterior third of the egg the blastoderm cells are deeper than else- 

 where, especially in the area where the germ band appears. At the 

 posterior end of the egg at the time of deposition a mass of symbiotic 

 plant cells are found. These become imbedded in a depression in the 

 thickened germ band. Above them in the plasma, which still separates 

 them from the yolk, a peculiar aster-like plasmic radiation develops (Fig. 

 175). The germ band now invaginates into the yolk, carrying at its 

 upper, open, or caudal, end the mass of symbionts which become enclosed 

 in an irregular syncitium formed by the germ band, the "aster" mean- 

 while degenerating. The break that existed at the place where the 



