220 EDWARD A. BOYDEN 



its maximum extent or how the movement of the line as a whole 

 across the neck is accomplished, is difficult to determine. One is 

 inclined to believe that the medial migration is an apparent 

 rather than a real movement, brought about by the addition of 

 new members to the medial end of the row, this end representing 

 an advancing growth-zone superimposed upon the advancing 

 zone of fusion between the operculum and the neck. In favor 

 of this hypothesis is the fact that the medial half of the row ex- 

 hibits the greatest solidarity, that the large medial filaments are 

 the ones that are usually branched (fig. 20), and that the lateral 

 members are the ones which drop out as the total length of the 

 line diminishes. There is the other possibility, however, that 

 the line as it stands is carried bodily inward, new filaments being 

 added laterally (the order of formation in the earliest stages), or 

 possibly interspersed among the old ones as the line is drawn out. 

 Following the period of maximum development during the sev- 

 enth day, the lateral members of the row gradually flatten out 

 until, as the line approaches the center, only a few of the medial 

 filaments on each side remain (fig. 9). By the beginning of the 

 eighth day both the opercular tubercles and filaments have been 

 absorbed into the neck. 



During this shifting of the rows of filaments from the sides to 

 the mid-line the under surface of the pectoral margin of the 

 opercular fold between the right and left zones of fusion has 

 given rise to a new line of filaments, — abortive structures which 

 are so small that they cannot be made out with the naked eye. 

 In fresh specimens, however, the margo pectoralis has that pearly 

 lustre characteristic of the marginal filaments. These abortive 

 filaments can just be made out with certainty in sections of 

 seventh-day embryos (H. E. C, Ser. 1950; 6 days, 2 hours; 16.0 

 mm.) (H. E. C, Ser. 2075; 6 days, 1 hour; 17.0 mm.), where 

 they appear as low sprouts of cells on the under surface of the 

 opercular fold. The largest of these are to be found on the tu- 

 bercles which lie on either side of the notch. As the marginal 

 filaments move in, they push this secondary line ahead of them 

 and at the end often form with these a confused tuft of cells just 

 prior to the final disappearance of both filaments and operculum 



