ORIGIN OF THE SPIRAL THREADS 



449 



endtr 



remaining but the moulted endotrachea, represented by the taenidia, 

 which lay loosely within the cavity of the trachea. The ectotrachea 

 or peritoneal membrane of the pupa is meanwhile in process of 

 formation ; the nuclear origin of the tsenidia is now very apparent. 

 Fig. 412 represents a longitudinal section through a secondary 

 trachea! branch, show- 

 ing the origin of the 

 chitinous bands, or tee- 

 nidia. At t' are pieces 



of six taenidia which ___ 



moulted; 



have been 

 ectr indicates the nu- 

 clei forming the outer 

 cellular layer, the ecto- 

 trachea or peritoneal 

 membrane. These nu- 

 clei send long slender 



FIG. 412. Longitudinal section of a trachea, showing the 

 prolongations around origin of the taenidia. 



the inside of the peri- 

 toneal membrane ; these prolongations, as may be seen by the figure, 

 become the tsenidia. The taenidia, being closely approximate, grow 

 together more or less, and a thin endotracheal membrane is thus 

 produced, of which the taenidia are the thickened band-like portions. 

 The endotracheal membrane is thus derived from the ectotrachea, 



or primitive tracheal mem- 

 brane, and the so-called " spiral 

 thread " is formed by thicken- 

 ings of the nuclei composing 

 the secondary layer of nuclei, 

 and which become filled with 

 the chitin secreted by these 

 elongated nuclei. The middle 

 portion of the taenidia, im- 

 mediately after the moult, is 

 clear and transparent, with 

 obscure minute granules, while 

 the nuclear base of the cell is 



fc 



en 



far 



ectr 



FIG. 413. Origin of the tonidia from nuclei. 



filled as usual with abundant granules, and contains a distinct 

 nucleolus. 



The origin of the taenidia is also well shown by Fig. 413, which 

 is likewise a longitudinal section of a trachea at the point of origin 

 of a branch. The peritracheal membrane or ectotrachea (ectr) is 

 2o 



