EMBRYONIC GROWTH OF THE 



131 



appears to be resolved into filaments, and all the secondary branches 

 assume the appearance of bundles of twine. Landois regarded tln-m 

 as the product of a transformation of the nuclei, but Gonin thinks 

 they arise from the entire cells, stating that from each cell arises 

 a ball (peloton) of small twisted tubes. 



As the large branches penetrate into the wing, the balls (pelotons) 

 of tine tracheal threads tend to unroll, and each of the new ramifica- 

 tions of the secondary tracheal system is accompanied in its course 

 by a bundle of capillary tubes. This secondary system of wing- 

 tracheae, then, arises from the mother trachea at the end of the third 



Fio. 144. Section through mesothorade segment of Datann tninixtru. passing throuirh the 

 wings (TI: <, cutii-iila : ti>/p. hvpodermis ; <ij>. apodenu- ; dm, dorsal longitudinal, ww, ventral 

 longitudinal, muscles: ihiit, depressor musHr cif tergum ; t, trachea; , nerve cords; /.intes- 

 tine ; n, urinary tubes ; /, insertion of legs. 



stage, when we find already formed the chitinous tunic, which Avill 

 persist through the fourth stage up to pupation. It differs from the 

 tracheoles in not communicating with the air-passage; it possesses 

 no spiral membrane at the origin, and takes no part in respiration. 



Gonin thus sums up the nature of the two tracheal systems in the 

 rudimentary wing, which he calls the provisional and permanent 

 systems. " The first, appearing in the second stage of the larva, com- 

 prises all the capillary tubes, and arising from numerous branches 

 passes off from the -lateral trunk of the thorax before reaching the 

 wing ; the second is formed a little later by the direct ramification 

 of the principal branch. 



