OLIGOENTOMATA AND APTILOTA 



187 



of its cells. As development continues, the serosa is drawn into the yolk 

 which in turn is gradually absorbed, so that at the time of the dorsal 

 closure only that remains which lies in the mid-gut. 



The germ cells are recognizable very early at about the time the germ 

 band begins to sink into the yolk. They form a mass of cells at the caudal 

 end of the germ disk, distinctly separated from the ectodermal cells. 



Fig. 110. — Lepisma. Longitudinal section, (am) Amnion, (br) Brain, (c. fit) 

 Caudal filament, (fg) Frontal ganglion, (ggl) Abdominal ganglion, (hyp) Hypopharynx. 

 (lb) Labium. Qim. m) Limiting membrane. Qr) Labrum. {prod) Proctodaeum. 

 ggl) Thoracic ganglion, (vise, ggl) Visceral ganglion. 



{th. 



They are also readily distinguished from the mesoderm cells by their 

 somewhat larger nuclei. Later the germ cells migrate, in a manner similar 

 to those of the Orthoptera, to the segmentally arranged genital ridges, five 

 pairs of which develop in the female and three double pairs in the male. 

 Terminal filaments are developed neither in the ovaries nor in the testes. 

 The development of stomodaeum and proctodaeum offers nothing 

 unusual. From the latter two pairs of small evaginations arise which 

 form the Malpighian tubules. The origin of the mid-gut, however. 



