212 EDWARD A. BOYDEN 



ficial resemblance to the filaments when seen in section. About 

 the same time a pair of sub-surface lines may be seen through 

 the translucent wall underlying each groove. A study of serial 

 sections (H. E. C, Ser. 2076; 6 days, 7 hours; 17.3 mm.; fig. 1) 

 proves them to be mesothelial ridges projecting into the peri- 

 cardial cavity. 



Hardly have these lateral grooves and ridges appeared than 

 they begin to shift their position from the sides of the embryo to 

 the mid-ventral line. This takes place in such a way that the 

 legs of the triangle first approach each other in front of the um- 

 bilicus and thereafter successively forward of that point, thus 

 resulting in an apparent ascent of the apex of the triangle. This 

 progressive movement is recorded on the median line by an 

 eruption of epitrichial cells which follows the retreating apex up 

 the pectoral wall until it reaches the surface ridges described 

 above. Thus a Y-shaped ridge is produced on the ventral sur- 

 face of the embryo the upper arms of which, the two surface 

 ridges, form a broad angle, and the lower arm of which, the epi- 

 trichial ridge, extends to the umbilicus. Micrometer measure- 

 ments of selected embryos indicate the rate at which the two upper 

 arms are coming together. In an embryo of five days, twenty 

 hours (17.5 mm.) the distance between the upper ends of the 

 ridges is 1.64 mm.; while in an older stage (6 days, 3 hours; 17.3 

 mm.) it has been reduced to 0.91 mm. ; and in a still older embryo 

 (6 days, 18 hours; 19.5 mm.) to 0.31 mm. The shifting of these 

 superficial ridges also keeps pace with the shifting of the opercu- 

 lum and filaments to be described later, so that if the ridges were 

 continued upward at any given stage they would strike the tufts 

 of filaments above. In all cases, however, the two structures are 

 separated by an appreciable area of the neck. Eventually the 

 surface ridges become heaped up in the median line thus con- 

 stituting, with the epitrichial proliferations, a continuous median 

 ridge from the umbilicus to the neck. In its upper end the evi- 

 dence of its paired origin is visible for some time and as a whole 

 the ridge persists for a number of days even to the time when it 

 becomes elevated upon the developing feather papillae (H. E. C, 

 Ser. 1967; 11 days, hours; 31.0 mm.). A similarly placed 



