218 EDWARD A. BOYDEN 



tion in the pectoral wall. But whatevi r interpretation they may 

 receive it is evident that they belong in a different category 

 from the filaments. 



LATER DEVELOPMENT OF FILAMENTS AND OPERCULUM 



The description of the early development of these structures 

 has been carried to the point where the opercula of the two sides 

 have united to form a swollen band of tissue across the ventral 

 surface of the neck (the plica opercularis) the posterior edge of 

 which (the margo pectoralis) slightly overlaps the pectoral wall. 

 On the lateral margin of the fold (the margo lateralis), a line of 

 filaments has been formed, which is separated from the one on 

 the other side by the whole width of the neck. This is the condi- 

 tion at the middle of the sixth day when the opercular fold has 

 reached its maximum length of two and a half to three milli- 

 meters. From now on, the neck will increase in diameter as the 

 fold undergoes reduction. This process consists in the fusion of 

 the under surface of the margo pectoralis with the ventral surface 

 of the neck, so that its form is changed from an overhanging 

 fold of tissue to a mound, which in turn flattens out and eventually 

 disappears. The striking feature of the whole process is that it 

 proceeds from the sides to the midline, at the exact rate and at 

 the same time that the pectoral ridges and marginal veins are 

 moving across the face of the pectoral wall. The rate of fusion 

 is easily gauged by measuring the decreasing distances between 

 the medial ends of the two rows of filaments as they are borne 

 along on the advancing wave. In all cases they move synchro- 

 nously with the structures below. Thus in an embryo of 5 days 

 and 20 hours (17.5 mm.) the unfused or overhanging portion of 

 the plica, measured by the distance between the filaments of the 

 two sides, is 1.45 mm. In the next seven hours it has been re- 

 duced in length to 0.55 mm. (embryo of 6 days, 3 hours; 17.3 

 mm.), and in the next fifteen hours to 0.23 mm. (embryo of 6 

 days, 18 hours; 19.5 mm.). The entire opercular fold including 

 both overhanging and fused portions of the two sides measure as 

 before some two and a half to three millimeters although the 

 fused portion is in process of sinking into the neck and disap- 



