No. 3-] LIMULUS POLYPHEMUS. 47 1 



tral side the cells are larger and gradually merge into the cells 

 composing the branchial cartilages. 



The ectodermic cells on the dorsal side of the embryo are 

 long and slender and arranged in rows, with their nuclei near 

 their outer ends. At the dorsal extremity of the cartilage there 

 is a point where the boundaries between these ectoderm cells, 

 the cartilage, and muscles merge together, and it is impossible 

 to distinguish between them (PI. XXIII, Fig. 34, x). 



A perichondrium is now visible, composed of a layer of spin- 

 dle-shaped cells, apparently derived from the mesodermic ring, 

 and not from a transformation of the peripheral cartilage cells. 

 The latter, as we have shown, are formed from the mesothelium 

 of the somatic walls. 



The distal ends of the branchial cartilages finally fuse with 

 the ectoderm that forms the anterior wall of the appendage. 

 At these points the cartilage and ectoderm are so completely 

 united that it is not possible to distinguish their original bound- 

 aries. Thus, an appearance is produced that might easily mis- 

 lead one into believing that the cartilages were growing as 

 inward proliferations from the ectoderm. But, aside from the 

 fact of their union at these points, there is no evidence that 

 such is the case. 



The spaces in the distal ends of the appendages are crossed 

 by fibrous columns ari'sing from the ectodermic walls. At the 

 base of each column are several nuclei, as though the columns 

 were formed by the union of several cells. No mesoderm 

 extends into the appendages beyond the distal ends of the 

 cartilages. 



T^e Abdominal Muscles. — Distinct muscles are first seen on 

 the dorsal wall of the somite of the operculum and the first gill 

 in specimens in which one gill leaf is developed (PL XXIII, 

 Figs. 25-29, m.). They increase rapidly and grow into the 

 yolk, forming the dorso-ventral muscles of the adult. There 

 are also stout bands of longitudinal muscles which extend from 

 the anterior end of one somite to the posterior end of the pre- 

 ceding somite. All the mesoderm cells of the mesodermic 

 ring are apparently converted into muscles attached to the 

 base of the abdominal appendages. 



