474 PATTEN AND HAZEN. [Vol. XVI. 



appear to be leaving the central mass and moving along the 

 walls of the somite towards the dorsal side of the Q.gg. Some 

 of them may possibly give rise to the nephric cells, which at a 

 much later period are found around the pericardium. 



The above description applies to the nephridial cells of the 

 fourth leg ; but a similar series of changes has also taken place 

 in the chelicerae and the second and third legs. At a corre- 

 sponding place in the fifth leg the duct of the nephridia has 

 been developing. The true nephridial cells do not appear in 

 that appendage until later. A longitudinal section (PI. XXIV, 

 Fig. 40), a little to one side of the median line, shows these 

 bunches of developing nephridial cells. In the fifth leg the 

 section passes through the end sac of the nephridial duct. In 

 the sixth leg there is a cavity similar to the end sac, but it is 

 not in the plane of the section. 



The nephridial cells of the fourth and fifth leg are shown 

 on a much larger scale (x 400), in a longitudinal section, in 

 PI. XXIV, Fig. 41. Here the cells are crowded with large 

 spherical granules that stain an intense blue in Lyon's blue. 

 The cell walls are often very faint and easily overlooked. A 

 few of these cells are seen in the fifth leg on the ventral wall 

 of the end sac. The walls of all the thoracic somites have 

 broken down and disappeared, except those of the fifth somite, 

 which is now more clearly outlined as the end sac of the nephric 

 duct {e.s.). 



In a longitudinal section of an embryo, just before the 

 Trilobite stage (PI. XXV, Fig. 43), the segmental arrangement 

 of the nephric cells is still clearly shown. The cells are rela- 

 tively larger than before, and the granules are breaking up into 

 smaller ones (PI. XXIV, Fig. 41). During the Trilobite stage a 

 marked change occurs in the character of the nephridial cells. 

 The oldest ones have elongated and become irregularly cylin- 

 drical, the coarse granules have disappeared, and the finely gran- 

 ular protoplasm has collected around the periphery. The nucleus 

 has also taken up its position just inside the cell wall (PI. XXV, 

 Fig. 45, n.c. and g.n.c). These hollow elongated cells then unite 

 end to end, forming a loose network of branching tubules 

 (PI. XXV, Fig. 47, n.c). 



