LIMULUS POLYPHEMUS. 



493 



INDEX LETTERS TO PLATE XXIV. 



EXPLANATION OF PLATE XXIV. 



Figs. 35-39 were drawn from cross-sections through corresponding regions of 

 the third and fourth thoracic appendages of embryos of varying ages. The sec- 

 tions are all arranged so that the median line is at the upper margin of the plate. 

 The sections of the younger embryos are 3 m to 5 ^ thick, the older ones 10 fi. 



Fig. 35. Cross-section through the middle of the fourth appendage of an 

 embryo in which none of the abdominal appendages had been formed. On the 

 dorsal margin of the mass of mesoderm lying near the median side of the base of 

 the appendage may be seen four or five larger cells, the " Anlage " of the nephrid- 

 ial lobe («.<;.). X 200. 



Fig. 36. Cross-section through the middle of the third appendage of an 

 embryo slightly older than that in Fig. 35. The nephridial cells are more numer- 

 ous, larger, and have faint granulations. A thin non-cellular membrane covers the 

 yolk at the base of the appendage. A space is formed between the mesoderm and 

 the apex of the appendage, which later develops into the blood space of the legs. 

 X 200. 



Fig. 37. Cross-section through the middle of the fourth appendage of an em- 

 bryo older than that in Fig. 36. The somites are imperfectly formed in the 

 thoracic appendages. The somatic layer is several cells thick ; the splanchnic 

 layer is represented by a thin membrane with a few nuclei. The nephridial cells 

 are larger than in the preceding figures. They possess slender pseudopodia, 

 have become finely granular, and take a deep stain in Lyon's blue. X 200. 



Fig. 38. Drawn from the same series and the fourth section back of that in 

 Fig. 37. It shows the nephridial cells smaller than at the middle of the base of 

 the appendage and without pseudopodia. The somite extends out laterally as a 

 closed cavity^ x 200. 



Fig. 39. Cross-section through the middle of the fourth appendage. Large 

 granular cells with long processes are shown on the dorsal margin of the meso- 



