166 WM. A. KEENER. 



the stigmatic process there is a cuticular and hypodermal in- 

 vagination which extends to near the base of the stigma as a 

 retaining thread (Fig. 8, inv.). This retaining thread of cuticle 

 and hypodermal epithelium is seen in transverse section at -inv. 

 in Fig. 9. The entire stigma represents a modified region of 

 hypodermis and cuticle. On the mesial side of the stigma near 

 the base of its anterior third the hypodermis becomes very 

 pronounced, the cells becoming very large and columnar. These 

 cells, so far as their form is concerned, are the most conspicuous 

 tracheal cells (Fig. 7, te.). From them slender processes go into 

 the cuticular mass of the stigmatic process. These processes 

 and the position of these cells suggest that they not only help to 

 elaborate the cuticular substances of the stigmatic process but 

 that, also, they may be able to move the stigmatic process. 

 Within the mesial wall of the stigmatic process no cytoplasm ex- 

 tends except that of these cellular processes; within the lateral 

 wall of the stigmatic process scattered hypodermal cells are 

 found. There is thus an indifferent cellular supply to the tracheal 

 process of the stigma. Indeed the entire stigma is for the most 

 part a cuticular structure. The cuticle of the general surface 

 of the body is distinctly two-layered. The outer layer is the 

 deeper and in hacmotoxylin stains the more deeply. The inner 

 layer is clearly a softer substance and does not stain deeply. 

 These two strata are involved in the formation of the anterior 

 stigma. The inner layer, except for becoming more abundant 

 in the stigma, is not modified. Figure 7 at c and Fig. 8 

 show this layer of the cuticle passing over into that of the 

 stigma. The outer layer of cuticle, however, is thinner over 

 the stigmatic process than over the general surface of the body. 

 When it reaches the tips of the papillae it is invaginated and passes 

 as a series of converging tubules to the bases of the papillae where 

 the tubules unite to form a large tube whose lining is confluent 

 with the lining of the tracheal trunk. The cuticular lining of 

 the tracheal trunk also presents a deeply staining layer and a 

 layer that does not readily stain (Fig. 11, //.), thus resembling 

 the cuticle, of which I believe it represents a modified region. 

 The inner denser layer of this tracheal lining gives rise to spiral 

 taenidia as shown in Fig. u at /. When this denser layer passes 



