No. 3] LIMULUS POLYPHEMUS. 473 



long after the duct, and the lobes in the other thoracic seg- 

 ments are clearly differentiated. 



Nephridial cells are first seen in the fourth segment at about 

 the time the somite appears. They appear a little later in the 

 third segment, and then in the second, first, sixth, and fifth in 

 the order named. 



The nephridial cells may be seen before the abdominal 

 appendages are formed, or before the somites are clearly out- 

 lined, on the dorsal wall of the mass of mesoderm at the base 

 of the appendages (PI. XXIV, Fig. 35, n.c). This mass of meso- 

 derm corresponds to the mesodermic ring of the abdominal 

 appendages, and later forms the somatic wall of the somite. 

 The splanchnic layer is at this period represented by a thin 

 non-nucleate membrane next the yolk. 



The nephridial cells may be recognized by their larger size, 

 their sharp nuclear stain, and by their lighter colored, trans- 

 parent protoplasm. They are best developed near the center of 

 the mesodermic mass at the base of the appendage, and from 

 that point they gradually merge in all directions into the sur- 

 rounding mesoderm. 



In the next stage (PI. XXIV, Fig. 36) the nephridial cells 

 are larger and more numerous, and some of the oldest ones 

 contain a few small granules. 



In the next stage (PL XXIV, Fig. 37) they have increased 

 greatly in size and numbers, and are very conspicuous, owing 

 to the deep blue color they take when treated with Lyon's 

 blue. The oldest cells have developed slender pseudopodia-like 

 processes. 



The membrane over the yolk contains distinct cells, forming 

 the splanchnic layer of the somites, and the latter are as dis- 

 tinctly differentiated as at any time in their development. They 

 do not at any time form closed sacs like the abdominal somites. 



The section shown in PI. XXIV, Fig. 38, passes through the 

 posterior margin of the fourth leg of the same embryo, and 

 shows the reduction in size and grade of development of the 

 cells on the periphery of the future nephric lobes. 



PI. XXIV, Fig. 39, represents a section through the middle 

 of the fourth nephric lobe in a still older stage. A few cells 



