MALPIGHIAN TUBES OF BOMBYX MORI L. 197 



convolutions. Finally, the posterior portion of each tube enters 

 the wall of the rectum and ends blindly after several convolutions. 

 The outer side of the tubes is richly provided with tracheae. 



For the sake of convenience, four regions may be distinguished 

 in describing in some detail the histological structure of the tubes : 

 a) the urinary bladder; h) the common stem; c) the coelomic 

 portion which lies freely in the body-cavity, and d) the portion 

 enclosed in the rectal walls, the 'included portion.' 



a. The urinary bladder. The urinary bladder is ellipsoidal 

 in form and measures about 0.95 mm. by 0.75 mm. in the fifth 

 stage. Its epithelium is composed of large flat cells, the cell 

 boundaries not always being clear. The cytoplasm, which stains 

 a violet-pink with eosin, shows a reticular structure as in figure 

 3. The nuclei are round or oval, and differ greatly in size, accord- 

 ing to the size of the cells. The nuclei are densely filled with 

 coarse deeply staining chromatin granules. The inner wall of 

 the epithelium is covered by a chitinous intima which is directly 

 continuous with that of the intestine. There is no striated 

 border, contrary to the description of Bordas ('11). The intima 

 is fairly thick and does not stain, with either eosin or haema- 

 toxylin. The basement membrane is a fine transparent struc- 

 tureless limiting membrane. In some sections it is not readily 

 detected. The circular muscles are strongly developed, and the 

 striations of the muscles are clearly defined, as is shown in figure 

 3. There are some longitudinal muscles interior to the circular 

 ones. 



b. The common stem. The common stem is a narrow and short 

 tube, measuring 0.52 mm. in length and 0.26 mm. in width. 

 It does not differ in appearance from the urinary bladder as 

 described above, excepting the smallness of the lumen, as is 

 shown in figure 5. The numerous cells at the distal end of the 

 common stem are very small and pressed closely together. These 

 cells represent the imaginal ring (fig. 1, ir), although they never 

 have a several layered appearance. The nuclei, which are round 

 or oval, are densely chromatic. The chromatin granules are 

 of the same color and size. During the larval stage the imaginal 

 cells divide to elongate the coelomic portion. 



