LIMULUS POLYPHEMUS. 



491 



INDEX LETTERS TO PLATE XXIIL 



B.C. — blood corpuscles. 

 Ch. = chelaria. 

 c.r. = cartilage rod. 



cu. = cuticular membrane. 

 ec.J>. = ectodermic proliferation. 

 ect = ectoderm. 

 end. = endothelium. 



^.' = first gill. 

 ^.(f. = genital duct. 



m. = muscle cells. 

 ms. = mesoderm. 



«. = nerve. 

 w./. = proliferation of nerve cells 



and fibers. 

 o.p. = operculum. 

 SO^ = somite of the chelaria. 

 SO^ = somite of the operculum. 

 SO^ = somite of the first gill. 

 v.s. = venous sinus. 



X. = fusion of cartilage, ectoderm, 

 and mesoderm. 

 jyk. = yolk. 



EXPLANATION OF PLATE XXIII. 



Figs. 21-24 represent longitudinal sections through the region of the chelaria, 

 operculum, and first gill of an embryo a little more advanced than the one from 

 which Figs. 9-20 in Plate XXII were drawn. The sections were 15/01 thick and 

 stained in borax carmine and Lyon's blue. 



Fig. 21. Section No. i, near the median line. The somites of the operculum 

 and the first gill have extended so far in a lateral direction that the somite of the 

 chelaria could not come in the same longitudinal sections with them, but would be 

 found in the sections nearer the median line. The somites of the operculum (<?./.) 

 and the first gill (^.') are large and distinct. The genital duct (^^.d.) shows at the 

 base of the operculum. The lumen seen here, ends in a solid mass of mesoderm 

 in the preceding section, x 200. 



Fig. 22. Section No. 5. The genital duct is larger than in the preceding sec- 

 tion, and has approached SO^. x 200. 



Fig. 23. Section No. ic. The genital duct and SO' have united. A few car- 

 tilage cells {c.r.) are seen on the dorsal wall of the somite, x 200. 



Fig. 24. Section No. 10. Both SO' and SO^ show a tendency to bend in a 

 posterior direction. The opercular cartilage {c.r.) is still present, x 200. 



Figs. 25-29. Longitudinal sections through the region of the operculum and 

 the first gill from an embryo with two gill leaves on the first gill. The sections 

 were 15 fj. thick and stained with borax carmine and Lyon's blue. 



Fig. 25. Section No. i, near the median line. The opercular cartilage and 

 genital duct extend toward the median line some distance beyond the somite. 

 The opercular cartilage is attached to the genital duct between bunches of meso- 

 derm at the base of the operculum. The first gill cartilage is now visible. In 

 the spaces between the ectoderm and mesoderm of the appendages are a few 

 blood corpuscles {B.c). Some of the mesoderm cells on the yolk show muscular 



