No. 3.] LIMULUS POLYPHEMUS. 489 



INDEX LETTERS TO PLATE XXIL 



a.p. — anal plate. 

 B.C. — blood corpuscles. 

 ch. = chelaria. 

 c.r. — opercular cartilage. 

 ect. — ectoderm. 

 g.' — first gill. 

 g.d. = genital duct. 

 op. = operculum. 



p./. = primitive furrow. 



p.s. = primitive streak. 



S0° = unexplained cavity appearing 

 only in this series. 

 SO^-^ = somite of the chelaria, opercu- 

 lum, and first to third gills, 

 respectively. 



EXPLANATION OF PLATE XXIL 



All the sections were outlined with a camera and drawn to the same scale. 



Figs. 1-8 were drawn from a series of longitudinal sections through the 

 region of the chelaria (cA.), operculum (op.), and the first gill {g.') of an embryo in 

 which the abdominal appendages were beginning to show in surface views. Borax 

 carmine, 1 5 ;w. 



Fig. I. Section No. i, through the median line of the embryo. A few 

 scattered mesoderm cells lie between the ectodermic layer and the yolk. There 

 were also a very few large nuclei in the yolk, and others from which chromatin 

 granules were escaping, as if they were beginning to degenerate, x 200. 



Fig. 2. Section No. 2. The median ends of the somites are showm by four 

 bunches of cells. SO^ is the somite of the chelaria ; SO^, that of the operculum ; 

 S0°, one which disappeared shortly after this stage ; SO^, somite of the first gill 

 about to separate from the primitive streak (p.s.). X 200. 



Fig. 3. Section No. 3. The somatic cavities are distinct ; S0° merges with 

 the ectoderm on the ventral side of the somite. X 200. 



Fig. 4. Section No. 6. SO^, SO^, and SO^ are larger than in the previous 

 sections. The cavity of S0° has disappeared, and in its place are a few mesoderm 

 cells which disappear in the next section, x 200. 



Fig. 5. Section No. 11. The operculum and the first gill are much larger 

 than formerly, and are now filled with mesoderm. Their somatic cavities remain 

 distinct. X 200. 



Fig. 6. Section No. 13. SO^ shows a diverticulum, the beginning of the 

 genital duct (g.d). x 200. 



Fig. 7. Section No. 18. SO^ remains large. SO^ has almost closed, x 200. 



Fig. 8. Section No. 23. SO^ is the only abdominal somite remaining ; its 

 lumen disappears a few sections farther toward the lateral side, x 200. 



Figs. 9-20 were drawn from a series of longitudinal sections through the region 

 of the chelaria, operculum, and the first gill of an embryo somewhat older than 

 the one in the preceding series. In this embryo there were four perfect abdominal 

 somites ; the fifth was just breaking free from the primitive streak. The series 



