PLANT-HOUSE ALEYRODES. I29 



bands a shallow median groove. Certain cells in the vicinity of 

 this groove seem to lose their epithelial nature and pass down- 

 v^ard until they come to be below the surface layer of cells. The 

 cells which go to make up the region of the groove pass down 

 and out to form a complete layer below the ectoderm. There is 

 no distinct separation of a median plate or tube in the formation 

 of the lower layer. The anterior end of the germ band becomes 

 widened out to form the cephalic lobes (Fig. 49). The germ 

 band is strongly flexed just posterior co the cephalic region, 

 so that the cephalic and thoracic regions and are nearly in appo- 

 sition. On the dorsal side of the expanded germ there is formed 

 quite early a slight invagination which marks the beginning of 

 the stomodseum. The proctodeeum is usually formed a little 

 later than the stomodaeum. 



Even before the formation of the lower layer is entirely com- 

 pleted the germ band begins to show traces of segmentation in 

 the thoracic and anterior abdominal regions. Very soon after 

 the segmentation of the germ band becomes apparent, the iirst 

 rudiments of the appendages begin to make their appearance. 

 The rudiments of the antennae arise first, being situated on the 

 cephalic lobes just posterior to the region of the stomodseum. 

 The rudiments of the mouth parts and of the thoracic limbs 

 appear simultaneously, and those of the abdomen appear suc- 

 cessively, from before backward, at a little later stage. The 

 abdominal rudiments have the form of little conical projections 

 on the ventral surface of the abdomen. They persist for a short 

 time only and, with the exception of perhaps the two posterior 

 pairs, leave no traces of themselves when they are absorbed. 



Tracheal invaginations begin to appear at about the same time 

 as the limb rudiments. Eleven pairs of invaginations are 

 formed ; — a pair on each thoracic segment and a pair on each of 

 the first eight abdominal segments. These invaginations, with 

 the exception of those on the thorax and on the eighth abdominal 

 segment, disappear during embryonic life. The four that persist 

 form the external openings of the tracheal system of the larva. 



The nervous system begins to appear simultaneously with the 

 tracheal system and the abdominal limb rudiments. The brain 

 is formed in the cephalic lobes as paired thickenings of the ecto- 

 derm just anterior to the stomodieum. The ventral chain of 

 ganglia are formed as ectodermal thickenings, a pair to each 



