476 PATTEN AND HAZEN. [Vol. XVI. 



enormous cells filled with refractive spherules. One large 

 spherule usually occupies the center, surrounded by many 

 smaller ones, so numerous as to completely hide the nucleus 

 (PI. XXVIII, Fig. 'J^). 



The division of the nephric lobes into concentric layers is 

 not a sharp one, still it is clearly evident on careful examina- 

 tions of the sections with moderately high powers. PI. XXV, 

 Fig. 48, was drawn on too small a scale to show these different 

 layers well. There can be no doubt that the nephric lobes from 

 now onward increase in size exogenously, and that the stratified 

 appearance of the lobes in section is due to a succession of 

 different stages of development that begins with indifferent 

 mesoderm cells on the periphery and ends with the fully 

 formed intracellular tubules in the center of the lobes. 



The nephridial lobes of the second, third, fourth, and sixth 

 leg resemble those in the fifth but the stratification of the 

 layers in the latter is more clearly marked. Cross-sections of 

 the stolons uniting these lobes also show very clearly the con- 

 centric strata. 



The difficulty of following the history of the nephridial cells 

 is much increased by the granules, which come and go, and 

 which change the appearance of the nephridial cells so much 

 that it is hard to recognize their various phases. They appear 

 to accumulate in the cells until the canalization of the latter 

 and their union to form a system of connecting tubules afford 

 the necessary means for their discharge into the end sac, and 

 from there to the exterior. It is certain that the granules begin 

 to diminish in numbers about the time the nephric duct acquires 

 an opening to the exterior. 



B. StrucUire of the Nephric Gland in the Adult. — In young 

 Limuli about two or three inches long the nephridial cells form 

 compact masses of tissue easily distinguished from the sur- 

 rounding organs. The cells at the base of the first and sixth 

 appendages have disappeared ; those at the base of the four 

 remaining appendages form the four permanent lobes of the 

 kidney. 



In the adult the lateral surface of each lobe, except the first 

 (PI. XXVIII, Fig. 83), is flattened and lobulated, with a roughly 



