LIMULUS POLYPHEMUS. 499 



INDEX LETTERS TO PLATE XXVIL 



ap? = fifth appendage. 

 ect.p. = proliferation of ectoderm. 

 e.n.d. — ectodermic portion of nephrid- 

 ial duct. 

 e.s. = end sac. 

 g.fi.c. = granular nephridial cells. 

 /.«.(/. = lip of the nephric duct. 



mes. — mesoderm. 

 m.r. = marginal ring. 

 u.c. — nerve cord. 

 n.d. — nephric duct. 

 p.n.d. = nephric plate. 

 s.m. ■= sphincter muscle. 

 so. — somite. 



EXPLANATION OF PLATE XXVIL 



Figs. 59-64 were drawn from a series of cross-sections through the fifth ap- 

 pendage of an embryo of the same age as that in PI. XXIV, Figs. 37 and 38. 

 The sections were cut 8 ju thick and stained with Delafield's haematoxylin. 



Fig. 59. Section No. i, showing a mass of mesoderm cells at the base of the 

 appendage, in the middle of which are a few larger and lighter-colored cells which 

 mark the anterior margin of the nephric duct, x 200. 



Fig. 60. Section No. 2, showing the nephric plate folded on itself, making a 

 double layer of large, clear cells, which extend toward the ectoderm. The somite 

 is a closed cavity dorsal to the nephridial duct, x 200. 



Fig. 61. Section No. 4, showing the nephric duct extending out to the ecto- 

 derm on the median margin of the appendage, x 200. 



Fig. 62. Section No. 6. The nephric duct and somite are much reduced in 

 size, but retain the same relative position as before, x 200. 



Fig. 63. Section No. 7, showing the somite reduced to a long, narrow space 

 on the surface of the yolk. In place of the nephric duct is a row of large 

 mesoderm cells with a slight outward projection, x 200. 



Fig. 64. Section No. 10. The nephric duct is represented by a row of large 

 cells, continuous on the median and lateral sides with the yolk membrane. The 

 mesoderm at the base of the appendage and the somite have entirely disappeared. 

 X 200. 



Figs. 65-70 are drawn from a series of cross-sections through the fifth 

 appendage of an embryo somewhat older than the one in the preceding series. 

 The proximal end of the duct has grown away from the median line, changing its 

 general direction somewhat. 



Fig. 65. Section No. i shows the anterior margin of the nephric plate, x 200. 



Fig. 66. Section No. 3 shows the mouth of the duct opening with a broad 

 lateral lip on the ventral side of the somite, x 200. 



Fig. 67. Section No. 5 shows cross-section through the middle of the duct. 

 X 200. 



Fig. 68. In Section No. 9 the lumen of the duct has disappeared, x 200. 



Fig. 69. Section No. 10, showing the distal end of the duct, x 200. 



