468 PATTEN AND HAZEN. [Vol. XVI. 



In the next series of sections (PI. XXIII, Figs. 21-24) the 

 genital duct is seen to open into the opercular somite, just 

 median to a transverse plate of mesoderm cells that will ulti- 

 mately develop into the opercular cartilage. 



In a still later stage, where one gill leaf has formed on the 

 posterior surface of the first branchial appendage (PI. XXIII, 

 Figs. 25-29), the ectodermic portion of the appendage extends 

 a considerable distance beyond the ring of mesoderm around 

 its base. Slender processes extend from the ectodermic cells 

 of one wall to those on the opposite side. In the space beyond 

 the mesoderm, blood corpuscles are frequently found. PI. XXIII, 

 Fig. 25, represents one of the sections from this series near the 

 median line, showing the genital duct {g.d.) directly under the 

 base of the cartilage. The cartilage has now grown out from 

 the ventral wall of the somite in a median direction over the 

 genital duct, and it also extends some distance ventrally into 

 the appendage. Following this series of sections laterally, the 

 somite is next seen at the posterior side of the genital duct 

 (PL XXIII, Fig. 26) with a band of muscle cells {m.) upon its 

 dorsal wall. The partition between the somite and the duct 

 becomes thinner, until finally the duct and somite unite, as 

 shown in PI. XXIII, Fig. 27. 



In the next stage to be described two or three gill leaves 

 have formed on the first branchial appendage (PI. XXIII, Figs. 

 30-32). The operculum is greatly elongated, and its opposite 

 walls are united by numerous outgrowths of the ectoderm. The 

 remnants of the mesodermic ring are still plainly visible at its 

 base. The genital duct is relatively smaller than in the preced- 

 ing stage and extends in a transverse direction along the dorsal 

 edge of the opercular cartilage. Its proximal end now loses its 

 connection with the opercular somite and remains for some time 

 in a rudimentary condition. In the second larval stage it can 

 be found at the base of the opercular cartilage as a short tube, 

 still unconnected with either the ectoderm or the coelom. 



In specimens half an inch long it was well developed and 

 united at its distal end with an infolding of the ectoderm. 

 The manner in which its inner end becomes united with the 

 genital organs has not been determined. 



