540 W. L. TOWER, 



but the crowding in the disc has caused a rearrangement of the 

 contents of the cell so that the nuclei lie at different levels with the 

 cytoplasm pressed out into the ends of the cell. Each cell reaches 

 from the inner to the outer side of the disc, which is then a single 

 layer of cells (and not many layered) (PI. 15, Figs. 16 and 17; 

 PL 17, Figs. 26 to 31 ; PI. 20, Figs. 64 and 65). 



During the evagination of the wing into the wing sac, or between 

 the hypodermis and cuticula, and through the larval stages until the 

 beginning of the prepupal stage the hypodermis of the wings changes 

 but little if any at all, the only marked change being the increase in 

 the number of cells (compare PI. 15, Figs. 16 and 17; PI. 16, Figs. 21 

 and 22; PI. 17, Figs. 26-31; PI. 20, Figs. 64-67). 



The hypodermis has on the proximal side a well developed 

 basement membrane which is made up of the anastomosed attenuated 

 prolongations of the basal ends of the cells, and a delicate layer of 

 mesoderm cells such as lines the proximal side of the hypodermis over 

 the entire body. There is no trace of other mesodermal cells taking 

 part in its formation as has been affirmed by several authors. The 

 hypodermal cells are thus bound together by this rather tough fibrous 

 basement membrane, so that when the cuticula is removed the hypo- 

 dermis presents a relatively firm structure, capable of resisting con- 

 siderable stresses without rupture. To this structure of the hypo- 

 dermis is due the capacity which it shows for the concentration or 

 extreme extension so often found during the metamorphosis of an 

 insect. In the development of the wing this membrane is of great 

 importance, for to it the hypodermal cells always remain attached 

 although the connecting portion may become extremely attenuated. 

 With the cells thus anchored to the basement membranes which fuse 

 to form the middle membrane, the two sides of the wing are held 

 near together so that the wing expands into a disc instead of a sac, 

 as it does when the membranes break apart or the connection of 

 the cells with the membrane is broken. It has been shown by Mayer 

 (1897), CoMSTOCK & Needhan (1899), Mercer (1900) and others that 

 these basement membranes fuse along certain lines to form the median 

 membrane. 



Near the close of the larval stage the wing begins to enlarge 

 rapidly and important changes occur in the hypodermis. At the be- 

 ginning of the prepupal stage the abdominal segments contract spas- 

 modically forcing the haemolymph anteriorly into the head and thorax, 

 where the pressure is partly relieved by being expended in dilating 



