OLIGONEPHRIDIA 



267 



abd.l 



The germ band now releases itself from the blastoderm, leaving behind 

 the grumulus ("polar organ" of Webster and Phillips), a cluster of cells 

 imbedded within a mass of protoplasm (Fig. 193). The amnion mean- 

 while has become thin and membranous (am), the attenuated blastoderm 

 forming the serosa (ser). As development proceeds the germ band 

 becomes much flexed (Fig. 194), the anterior and posterioi ends lying 

 nearly in contact with each other, both 

 pointing toward the posterior pole. By 

 this time the germ layers have differ- 

 entiated, the entoderm being thin and 

 less compact than the ectoderm, form- 

 ing an almost continuous sheet over the 

 inner surface of the germ band. At 

 this stage the body segmentation be- 

 comes apparent, and the mycetom 

 assumes a more anterior position in 

 relation to the embryo. Between the 

 mycetom and the germ band a group of 

 cells that have apparently separated 

 from the mesoderm give rise to the 

 gonads (Fig. 194, ov). The primary 

 yolk divides into yolk spherules, each 

 with one or more yolk cells. The 

 embiyo next changes its position so that 

 when viewed from the side it has the 

 form of the figure 6 with the stem 

 directed toward the posterior end of the 

 egg. The mycetom now lies in the 

 region of the first abdominal segment 

 between the dorsal side of the embryo 

 and the yolk, and just behind it is the 

 spherical gonad. Cephahc, thoracic, 

 and abdominal regions are sharply 

 marked, each distinctly segmented, the 

 first with five, the second with three, and the last with nine segments 

 (Fig. 195). 



In the next stage the abdominal region changes its position by foldmg 

 back dorsally over the mycetom (Fig. 196). The proctodaeum has by 

 this time been formed as well as the stomodaeum, and the mid-gut is in 

 the course of formation above and resting on the mycetom. This is the 

 stage in which most of the embiyos pass the winter. 



In the spring the embryo advances in the yolk toward the posterior 

 pole until the amnion in the dorsocephalic region comes in contact with 



Fig. 195. — Toxoptera. Ventral as- 

 pect, {abd 1-9) Abdominal segments 

 1-9. (ant) Antenna, (lb) Labium. 

 (Ir) Labrum. (md) Mandible, (mx) 

 Maxilla, (p 1) First thoracic leg bud. 

 (stom) Stomodaeum. 



