No. 3-] LIMULUS POLYPHEMUS. • 475 



These changes begin first in the center of each segmental 

 mass of nephric cells, so that for a time each set of tubules is 

 unconnected with the tubules in the adjacent segments. 



Some nephric cells are still present like those described in 

 the earlier stages (PI. XXVIII, Fig. 80, c and d). On the 

 periphery of the lobes are small granular cells that stain deeply 

 in Lyon's blue, and which appear to be blood corpuscles {s.r.c). 



The formation of nephric tubules now begins to extend for- 

 ward and backwards, forming at first a few slender chains of 

 cells uniting the ventral ends of the nephric lobes with one 

 another. As these connecting cells become canalated, the 

 branching tubules in each lobe are united into one system. 

 The various kinds of cells seen in the nephridial lobes are 

 shown in PI. XXVIII, Figs. 74, 75, Ty, and 79. 



In a crab about one inch long a cross-section in the region 

 of the fifth leg (PI. XXV, Fig. 48) shows that the nephridial 

 lobes now consist of several distinct layers or strata, composed 

 of cells in different stages of development. The end sac 

 (e.s) on the median side opens through numerous tubules into 

 smaller ones, which again break up in the loose tissue of the 

 gland. The tubules in the region of the end sac are large, and 

 open with such wide mouths into the end sac that it is not 

 possible to determine just where the sac ends and the tubular 

 tissue begins. The layer of large tubules nearest the end sac 

 is lined with pavement cells filled with coarse granules. These 

 tubules are evidently formed from the hollow cell chains of 

 the previous stage by the multiplication of the nuclei and 

 the breaking up of the peripheral protoplasm into separate 

 cells. Thus, the intracellular chain of vacuoles is changed 

 into the intercellular lumen of a duct lined by many flattened 

 cells. 



The next layer is composed of chains of cells with finely 

 granular protoplasm and conspicuous nuclei. Most of the cells 

 are hollow, and united end to end to form a network of intra- 

 cellular tubules like those seen in the second larval stage 

 (PI. XXV, Fig. 47). 



The outer layer or cortex of the lobe consists of many small 

 cells with very conspicuous nuclei. Among them are some 



