210 EDWARD A. BOYDEN 



sessing two lateral margins, each fused to the side of the neck 

 and })ordered by a line of filaments, and a single pectoral inargin 

 whose free edge is directed posteriorly. A notch at its middle 

 point (incisura opercidaris) indicates the place where the two 

 arches originally united, and a tubercle on each side of the notch 

 still further accentuates the paired origin (fig. 1). In later stages 

 the free, overhanging, pectoral margin becomes more and more 

 encroached upon by the lateral margins, as its under surface pro- 

 gressively fuses with the surface of the neck, from the sides to 

 the mid-line. As these lateral zones of fusion pass slowly inward 

 they carry the lines of filaments with them so that these likewise 

 come to lie successively nearer the mid-line. Meanwhile the 

 pectoral wall below the opercular fold has developed a pair of 

 surface markings of its own which begin to shift their position 

 from the sides to the mid-line at the same time and rate as the 

 lines of filaments and zones of fusion above. Eventually these 

 markings come to form part of a median ridge extending from 

 the region below the operculum to the umbilicus. In addition to 

 their migration these pectoral markings or ridges exhibit a fur- 

 ther, albeit superficial, resemblance to the filaments in that the 

 cell-proliferations of which they are made often contain scattered 

 degeneration vesicles and pycnotic nuclei, but to a much lesser 

 extent than obtains in the branchial epithelium. Because the 

 pectoral ridges and lines of filaments are thus found to possess 

 these common features it becomes necessary to establish the 

 identity or the disparity of the two sets of structures, the one on 

 the neck and the other on the pectoral wall, — hence the following 

 digression. ^ 



CHANGES IN THE PECTORAL WALL CORRELATED WITH THE LATER 

 DEVELOPMENT OF FILAMENTS AND OPERCULUM 



During the latter part of the sixth day and the beginning of 

 the seventh, four different sets of structures make their appear- 

 ance in the pectoral wall, all of which are represented in figure 1 : 

 a pair of pectoral grooves {sulci peciorales) ; 2) a pair of pectoral 

 ridges (cristae pectorales) ; 3) a pair of mesothelial ridges (cris- 

 iae mesoiheliales) and 4) a median epitrichial ridge {crista 

 epitrichialis) . 



