HARDWICKE'S SCIENCE-GOSSIP. 



205 



absorb colouring matter with difficulty. The thread, • 

 from its greater thickness, offers a longer resistance 

 to solvents, such as caustic alkalies, and also to 

 mechanical force. It can therefore be readily 



divided into polygonal areas, each of which is 

 occupied by a reticulation of very fine threads. This 

 structure may be traced for a short distance between 

 the turns of the spiral thread. 



Fi:j. 120. — Tracheal System of Cockroach. Top and front of head seen from 



without. X 15. 



Fig. 119. — Tracheal System of Cockroach. The 

 ventral integument and viscera removed to 

 show dorsal tracheal communications. X 5. 



Fig. 121.— Tracheal System of Cockroach. Back of head, seen from the 

 front, the fore half being removed. X 5. The letters A— J indicate 

 corresponcing branches in figs. 120, 121, and 122. 



unrolled, and often projects as a loose spiral from 

 the end of a torn tube, while the membrane breaks 

 up or crumbles away. 



The chitinogenous Isyer of the tracheal tubes is 

 single, and consists of polygonal, nucleated cells, 

 forming a mosaic pattern, but becoming irregular and 



The large tracheal tubes close to the spiracles are ' even branched in the finest branches. The cell-walls 

 without spiral thread, and the intima is here sub- ; are hardly to be made out without staining. 



