3o6 THE DEVELOPMENT OF THE NYMPH. 



epiblast takes place slowly. The paraderm of the thoracic region 

 is removed during the rapid increase of the thoracic discs, 

 which grow over its surface and subsequently unite with each 

 other except on the dorsum ; where the paraderm remains, 

 however, the only covering of the pseudo-yelk long after 

 the complete evolution of the head. 



When the head is thrust forwards from the interior of the 

 thorax, the cephalic discs are still in a very rudimentary con- 

 dition, and are not united with each other except by the 

 paraderm. Van Rees ascribes the evolution of the head and 

 thorax to the contraction of the muscles of the larva before 

 their final degeneration, but it is certain that these are in an 

 advanced state of histolysis, and have lost all connection with the 

 integument before it occurs. The evolution of the head and 

 thorax is not, therefore, the result of muscular contraction, but 

 of the organic changes which take place in the cells of the para- 

 derm. These are slow and continuous, and the contraction of 

 its surface, accompanied by increase in its thickness, continues 

 until it is finally replaced by the ectoderm of the discs. 



e. Histolysis of the Tracheae of the Larva and Development of the 

 Tracheae of the Pronymph. 



The tracheal vessels of the pronymph are developed from the 

 anterior superior thoracic disc, which surrounds the spiracular 

 trunk of the larva, and from the vessels already described 

 (p. 85), which exist in the larva in relation with the imaginal 

 discs, and exhibit an outer coat formed of small embryonic 

 cells. 



The greater part of the larval trachese undergo active histo- 

 lytic changes; the external cellular coat is entirely stripped 

 from them by the action of phagocytes, and the naked 

 intima collapses. 



The intima of the great longitudinal trunks is seen for three 

 or four days lying in the pseudo-yelk. As Weismann observed, 

 it is severed from its connection with the persistent portion of 

 the tracheal system very near the anterior spiracle. It con- 

 tracts to about half its original length, lies entirely in the 



