OLIGONEPHRIDIA 



253 



symbionts were located in the germ band at the caudal junction of 

 the germ band and amnion closes when the sj^mbionts, enclosed in their 

 syncitial envelope (mycetom), pass out of the germ band into the yolk 

 at the anterior end of the egg. The germ band meanwhile is reflexed at 

 its caudal end (Fig. 176). The amnion, at first thick and indistinguish- 

 able from the germ band, becomes a thin membrane. The part of the 

 blastoderm not concerned in forming the germ band and the amnion 

 is the serosa, in which, owing to stretching of the membrane, the nuclei 

 lie far apart. The germ band broadens, acquires broad head lobes, and 



Fig 175 — Pediculus Sagittal section. Fig. 176. — Pediculus. Sagittal sec- 



(gb) Germ band, {myct) Mycetam. {aer) tion. (am) Amnion. (ect) Ectoderm. 

 Serosa. igb) Germ band, (mes) Mesoderm. 



{myct) Mycetom. {ser) Serosa. 



then separates from the serosa. The mycetom now lies wholly free in 

 the yolk (Fig. 177, myct) under the micropyle. At the time of invagina- 

 tion of the germ band, the cleavage of the yolk into spherules begins, first 

 at the posterior pole, then at the anterior pole, and finally in the center. 

 Vitellophags within the spherules reduce the yolk to a homogeneous 

 mass. 



When the germ band has become flexed (Fig. 176), the inner layer 

 begins to develop on its dorsal, or yolk, side in the form of isolated cell 

 islands along the median hne. These cell islands enlarge and finally fuse 

 into an inner layer which represents the entoderm and mesoderm. 

 While the inner layer is forming, first the proctodaeum (Fig. 177) and 

 then the stomodaeum make their appearance. The anterior and pos- 

 terior mesenteron rudiments are derived from the inner layer (Fig. 180). 



