C Journal New York Entomological Society. [v<,i. vin. 



in\agination. At a point two or three sections distant from the cen- 

 ter, the invagination is closed in by the hypodermis of the body-wall 

 (Fig. 6, /'). The opening of the invagination is, therefore, a narrow- 

 pore. 



The single layer of the hypodermis appears to pass into two or 

 more layers in the wing-bud, *but upon a careful examination it will 

 be seen that there is but one layer, the nuclei of the cells being at 

 different levels, while the cell walls may be traced from one side of 

 the wing-bud to the other. In this stage, at the point of nearest ap- 

 proach of the trachea to the wing, there is a slight depression and the 

 cells of the trachea are elongated slightly on the side toward the de- 

 pression (Fig. G). 



Different specimens of larwe in the third stage exhibit a great differ- 

 ence in the appearance of the basement meml)rane of the hvpodermis 

 of the body-wall. In some it is distinguished with difficulty, being 

 merely a line limiting the inner ends of the hypodermal cells ; which, 

 in this case, are the same diameter throughout their length. In others 

 the inner ends of the hypodermal cells are prolonged into delicate fibers 

 between which are spaces ; in this case the basement membrane con- 

 necting the tips of their fibers is remote from the main body of the cell 

 (Fig. 6(7, /-'///). In both cases the basement membrane of the wing- 

 bud, which is directly continuous with that of the hypodermis of the 

 body-wall, appears merely as a line bounding the ental surface of the 

 bud. The tissue of the bud is solid, without the spaces represented 

 in Figure 6c7. It is, therefore, difficult in some specimens to dis- 

 tinguish the basement membrane of the wing-bud in this stage. 



ThkFourtfi Larval Stage. (Plate I, Figs. 7-11 : Plate II, Fig. 15.) 

 Sections of a larva taken in this stage (Figs. 7 and 8) show that 

 the process of invagination has been going on at a rapid rate. The 

 slight depression in the region of the trachea in the third stage (Fig. 

 6) has increased to a prominent evagination. It will be observed 

 that in the course of this internal development of the wing there is 

 first an invagination of the hypodermis, and later an evagination of a 

 portion of this invagination. The evaginated portion is what is 

 destined to become the wing ; while the remaining, thinner portion of 

 the invagination forms the peripodal membrane. 



It is in this stage that a temporary set of tracheoles is developed. 

 The cells of the epithelium of the large trachea near the wing-bud 



