BRANCHIAL CARTILAGES OF LIMTM'S. 309 



than the surrounding mesoderm cells, and have distinct cell walls; 2, they are 

 arranged in rather regular order; and 3, the protoplasm stains very lightly in 

 borax carmine. 



In the next stage, with three gill leaves on the first branchial appendage (Fig. 

 210, D), the cartilages form long flat plates that extend some distance beyond 

 the ring of mesoderm into the appendage. The cartilage of the first gill is 

 attached to the anterior wall of its appendage, and extends from there to the 

 corresponding somite, which has now become a venous sinus bounded by a thin 

 membrane. 



A perichondrium is now visible, composed of a layer of spindle-shaped cells, 

 apparently derived from the breaking up of the mesodermic ring, and not from a 

 transformation of the peripheral cartilage cells. The latter, as W T C have shown, 

 are formed from the mesothelium of the somatic walls. 



The ends of the branchial cartilages finally fuse with the ectoderm on the 

 anterior wall of the appendages and with the haemal wall of the abdomen. At 

 these points the cartilage and ectoderm are so completely united that their original 

 boundaries cannot be distinguished. 



The spaces in the distal ends of the gills are crossed by fibrous columns 

 arising 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. At this stage 

 no mesoderm extends into the appendages beyond the distal ends of the cartilages. 



In the early trilobite stage the branchial cartilages differ but little, except 

 in size, from those in the adult. 



Minute Structure of Branchial Cartilage. During the late larval period, 

 each cartilage cell develops on its outer surface a cartilage-like matrix that en- 

 velops and isolates each cell. This is the primary capsule. It gradually in- 

 creases in size, forming a large thin-walled chamber, within which the cell con- 

 tinues to divide in the three planes of space, each division plane being generally 

 at right angles to the preceding one. After each division the daughter cells form 

 new capsules inside the old ones. As there is little or no shifting of the cells 

 after each division, the shape and position of the primary, secondary and tertiary, 

 etc., capsules show pretty clearly the history of the previous divisions. 



The cluster of cells and capsules enclosed in the primary capsule constitutes 

 a cartilage nest. The nests are largest in the axis of the gill bar; the periphery 

 of the bar consists of small cells not clearly grouped into nests. The cartilage 

 tissue terminates abruptly under the perichondrium. The two tissues are dif- 

 ferent chemically and ontogenetically and there are no indications of intermediate 

 stages between them, although occasionally one finds a small cluster of cartilage 

 cells enclosed like a foreign body, in the perichondrium, or in the fibrous tissue 

 connecting the inner ends of the entopophyses. Similar nests of capsuligenous 

 cartilage are said to occur in the endocranium of Mygale (Lankester) and Tele- 

 phonous (Gaskell). 



When the gill bars with their adhering fibrous and muscular tissue are boiled 



