447 



developed their remarkable filamentous structure; others are 

 in process of dividing. This leaves the cavity, in which they 

 lay, devoid of cells, and at this stage a canal, formed by the 

 incomplete fusion of the bases of the large cells, is developed 

 right along the axis of the gland, and opens below into the 

 large basal chamber. The great elongated cells have mean- 

 while begun to fuse on their outer surface, and from now 

 on the rectal gland may be divided into an outer syncytial 

 cortical portion, surrounding an inner medullary region in 

 which cell walls are still well marked. The cortical portion 

 is formed by the ifusion of the outer ends of the elongated 

 cells; the medulla is the region which becomes differentiated 

 by the failure of the cells to fuse here. 



In the thirty-six hour pupa the glands have grown to their 

 maximum size. Cortex, and meduUar~*re~-very elearly seen ^ 

 (fig. 163). The central canal is prominent; the basal cells ' 

 have differentiated into their adult filamentous condition. 

 The cells of the rectum at the base of the two glands have 

 proliferated a little to form two thickened pads; a trachea 

 soon penetrates the rectal wall here. During the third day, 

 the thickened pad begins to chitinise. 



Even in the fifty-six hour pupa the medullary region is 

 distinguishable, but from now on the syncytium becomes more 

 and more developed, and in the mature organ no distinction 

 can usually be drawn between medulla and cortex. 



In the advanced pupa the large elongated cells begin to 

 develop nucleoli of extraordinary dimensions (fig. 166). Some- 

 times they occupy nearly the whole nuclear space. 



The Malpighian Tubes. 



In the adult larva shortly after the cessation of feeding, 

 the malpighian tubes become visible as small papillae on the 

 anterior end of the hindgut (fig. 160). Here the cells of the 

 imaginal ring have begun to develop, and it is not till this 

 time that definite malpighian tubes can be observed. 



The papillae begin to grow with extraordinaary rapidity, 

 and by the time the larva defeacates (i.e.^ twelve hours later) 

 they are visible as long thin threads (fig. 151), sometimes 

 reaching almost to the dorsal body surface. From the first 

 they have a narrow lumen. Tlie walls are one cell in thick- 

 ness; the cells are roughly cubical and fit loosely together. 

 The tubes measure about 6/x in diameter. 



The tubes continue to grow in length, but not in thick- 

 ness. In the fresh pupa the cells, which had previously the 

 usual embryonic features — ^large karyosome, "vesicular" 

 nuclei, hyaline cytoplasm — now begin to show signs of dif- 

 ferentiation. The nuclei become granular, and the cytoplasm 



