500 PATTEN AND HAZEN. 



Fig. 70. Section No. 11, showing the union of the ectodermic invagination 

 with the mesodermic portion of the duct, x 200. 



Fig. 71 is a longitudinal section through the middle of the fifth appendage in 

 the early Trilobite stage, showing the opening of the nephric duct to the exterior. 

 X 200. 



Fig. 72. Section on the lateral side of the base of the fifth appendage in a 

 late Trilobite stage. It shows the end sac, which extends beyond the base of the 

 appendage on the surface of the yolk. Granular cells (^.<r.) are developing on its 

 ventral margin. X 200. 



Fig. t^. Longitudinal section through the fifth appendage of a specimen 

 somewhat older than that of the preceding figures. It shows the distal arm of the 

 nephridial duct and its opening at the posterior side of the appendage. The ecto- 

 dermic portion is characterized by numerous small cells, which at the very end 

 seem to be forming a sphincter muscle around the opening. The mesoderm cells 

 are large, with a faintly colored protoplasm. In many specimens a lumen was 

 continuous throughout the mesodermic and ectodermic portions, x 300. 



