352 HISTOLOGY 



The second part of the nephridial tubule is narrower than the con- 

 voluted portion; its diameter is about forty-five microns. The epithe- 

 lium of the wall is composed of very low columnar cells which tend to be 

 wider than tall. The cytoplasm is denser than that of the convoluted 

 portion, and the cells have nuclei slightly smaller than those of the larger 

 part of the tubule, but which have the same general structure. This 

 smaller portion of the tubule most probably serves much less as an 

 excretory structure than the convoluted portion. The smaller part 

 of the tubule leads to and empties into a collecting tubule. The latter 

 has an origin independent of the nephridial tubule. 



In an embryonic stage it meets the nephridial tubule, and its wall 

 coalesces with the wall of that structure. In Figure 315 we have shown at 

 A a place of union between these tubules. The section unfortunately 

 shows the cells cut obliquely so that parts of cells cut above their nuclei 

 are seen lying to the right of the nucleated portions. Though the section 



FIG. 315. A, section through the point of union of the epithelium of a collecting tubule and a 

 nephridial tubule in the Iguana's kidney; B, some epithelium from the collecting tubule. 

 X 870. 



is not in this respect ideal, we may see the nuclei of the nephridial tubule, 

 with their characteristic structure, interspersed with those of the collecting 

 tubule. The cells of the collecting tubule become taller as they leave 

 the nephridial tubule. At B in Figure 315 they are shown, seen in pro- 

 file. The cytoplasm is finely granular and very clear at the distal end. 

 At the base, and in some cases along the sides, a layer of denser cytoplasm 

 occurs. The nuclei are dense and stain deeply. They are but eight or 

 nine microns in diameter. The nuclei always lie near the base of the 

 cells. These cells closely resemble mucous cells, while the cells of the 

 nephridial tubule are suggestive of serous cells. 



Examples of nephrostomes. The nephrostome of Polygordius is 

 comparatively simple. The intracellular lumen opens at the distal 

 end of the nephridial tubule into the body cavity. At one margin of 

 this opening is borne a tuft of cilia which is quite suggestive of the 

 "flame" of a "flame-cell" (Fig. 316). 



In Perich&ta malamaniesis Benham, the nephrostome "consists of 

 eight or nine marginal cells set in a circle around the terminal aperture 



