Mar. I900.J MeRCER : DEVELOPMENT OF WiNGS IN LEPIDQPTERA. T 



(Fig. 7, T) are elongated on the side toward the evagination of the 

 wing (Fig. 7, met). A little later in this stage these cells appear 

 very much larger \vith large nuclei (Fig. 8, met). These cells are 

 destined to form the tracheoles ; and may be termed the mother cells 

 of the tracheoles. In some of these cells, very fine tubes are seen, 

 through which the light passes in the section. This is the first ap- 

 pearance of the temporary respiratory organs of the wing, or the 

 tracheoles. Later in this stage the cells have wholly disappeared 

 except the nuclei, and in their place are great bundles of capillary air 

 tubes, the tracheoles, which difler from tracheae in lacking the spiral 

 thickening of the intima (Fig. 15, //). The nuclei pf these mother- 

 cells of the tracheoles do not disappear until after the development 

 of the tracheoles (Figs. 10 and 11). It is evident from this that the 

 tracheoles are not formed, as Landois supposed, from the nuclei of 

 the cells, but from the body of the cells as described by Gonin. 

 Figure 1 1 shows a group of cells in cross section, very much enlarged. 

 In two of them the nucleus is cut through and appears, while in the 

 other two the razor has not passed through the nucleus. In cut.ting 

 across a group of cells containing a bundle of tubes, the section 

 would show round holes, which is the case with these cells. Two 

 other cells (Fig. 10) are shown in longi-section. These show the 

 nuclei with a bundle of tubes surrounding them. 



During this stage the tracheoles have no communication with the 

 lumen of the large trachea about which they are developed. But as 

 the new intima (Fig. 15, nin) is formed, which is to line this trachea 

 in the next stage, it is not extended over the mouths of the tracheoles. 

 It will be seen, therefore, that on the removal of the old intima (Fig. 

 15, oin:) af the following moult, the air in the lumen of the trachea 

 will have free access to the mouths of the tracheoles (Fig. 15, mt) 

 which now become functional. 



Proliferation of the Traelieee.—.^i the very beginning of the fifth 

 stage at the latest, and there is considerable evidence that in the latter 

 part of the fourth stage, there begins the development of the permanent 

 wing-trachese. These arise as a pocket-like evagination of the epithelial 

 walls of the trachea that gives birth to the tracheoles. This is shown 

 diagrammatically in Figure 9, ef. This evagination is increased in 

 size, and others are formed, some of which branch andextend into the 

 lumen of the wing, following in a general way the course of the 

 bundles of tracheoles. Figure 9 is diagrammatic to show more clearly 



