470 PATTEN AND HAZEN. [Vol. XVI. 



In the next stage, where one gill leaf is developed on the 

 first branchial appendage (PI. XXIII, Figs. 25-29), the oper- 

 cular cartilage has grown rapidly ; its ventral edge projects 

 into the mesodermic ring, but the median and lateral sides are 

 still continuous with it. 



In the preceding stage (PI. XXII, Figs. 15-20) the cartilage 

 extended only from the point of union of the genital duct with 

 the somite to the lateral margin of the mesodermic ring. In 

 this stage (PI. XXIII, Figs. 25-29) it has grown in a median 

 direction along the ventral wall of the genital duct, so that the 

 two structures are for a short distance completely fused with 

 each other (PI. XXIII, Figs. 25 and 26, cr). 



The cartilage cells now begin to show the features that char- 

 acterize them so clearly in the later stages; viz. : (i) they are 

 larger 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. 



The first gill cartilage shows a very similar condition to that 

 in the operculum. 



In the next stage, with three gill leaves on the first branchial 

 appendage (PI. XXIII, Figs. 30-33), the cartilages form long 

 fiat plates that extend some distance beyond the ring of 

 mesoderm into the appendage (PL XXIII, Fig. 33). The car- 

 tilage of the first gill is attached to the anterior wall of its 

 appendage and extends from there to the somite at its base. 

 A section nearer the lateral line would show a similar attach- 

 ment of the opercular cartilage to the anterior wall of its 

 appendage. The cartilage cells are of uniform size and have 

 a characteristic appearance and arrangement in rows. The cell 

 walls are sharply outlined against the clear homogeneous proto- 

 plasm. The nuclei are large and oval and deeply stained. The 

 cartilage is surrounded by a thick hyaline membrane, which is 

 continuous with the outer wall of the somite. 



In the early Trilobite stage (PL XXIII, Fig. 34) the carti- 

 lages differ but little, except in size, from those in the adult. 

 Each abdominal somite has now become a venous sinus bounded 

 by a thin membrane. The dorsal wall of the sinus is composed 

 of small oval cells, which take a very deep stain. On the ven- 



