No. 3-] LIMULUS POLYPHEMUS. 479 



the early Trilobite and second larval stages. These cells and 

 tubules represent the peripheral terminations of the system of 

 tubules leading into the end sac. This layer nearly surrounds 

 the lobe. It is thickest at its apex, becomes thinner on the 

 median ventral side, and disappears entirely on the median 

 dorsal side, where the longitudinal collecting tubes unite the 

 lobe with one another. Within this layer is one formed of 

 tubules, lined with large granular cells, as shown in PI. XXVI, 

 Fig. 49, g.t., and PI. XXVI, Fig. 52. They are surrounded by 

 a loose connective tissue, containing nuclei larger than those in 

 the granular cells. There are two kinds of nuclei in the walls 

 of the tubules, one small, dark, and homogeneous, the others 

 larger, and showing clearly the chromatin granules. 



In the next layer (PI. XXVI, Fig. 49, t.p., and PI. XXVI, 

 Fig. 53) the cells have lost their granules and have flattened 

 out to form a thin endothelial lining to the tubules. The 

 tubules are large, and really form a meshwork of spaces 

 separated by vacuolated connective tissue. Blood channels, 

 containing large granular blood corpuscles, are abundant in 

 the connective tissue surrounding the tubules. The large col- 

 lecting tubules are best developed in the center of the lobe 

 and on the dorsal surface at their median ends. The endothe- 

 lium of these tubes (PI. XXVI, Fig. 54) stains more deeply, and 

 is vertically striated on its surface farthest from the lumen of 

 the tubes, next the very distinct basement membrane. The 

 tubes are widely separated by a spongy connective tissue, 

 richly supplied with blood vessels. 



VIII. The Nephric Duct, 



A. The Development of the Nephric Duct. — The nephric 

 duct develops as an evagination of the somatic mesoderm of 

 the fifth leg. The duct cells appear before the nephridial cells 

 of that segment, and before the boundaries of the somite are 

 clearly defined, as an oval plate of columnar cells, easily recog- 

 nized by their large size and clear protoplasm (PI. XXVI, Figs. 

 55-57). At the edges of the plate they pass gradually into the 

 undifferentiated mesoderm that covers the yolk. Beneath the 



