GILL-FILAMENTS IN SAUROPSIDA 21 & 



pearing. By this time the projected width of the neck at this 

 level measures some three and a half millimeters. In round 

 numbers, during the twenty-four hours following the maximum 

 development of the operculum the unfused portion has been 

 reduced by eighty per cent of its former length while the neck 

 has added twenty per cent to its circumference. By the begin- 

 ning of the eighth day the united opercula have become reduced 

 to a pair of tubercles on either side of the mid- ventral line which 

 are themselves on the point of being incorporated in tl^e neck. 



While the opercular fold has been undergoing a decline, the 

 filame ts have reached their maximum development and have 

 likewise entered a period of decline which is completed with the 

 disappearance of the tubercles. In the beginning it was stated 

 that the filaments were solid outgrowths of eels arising chiefly 

 from the ectoderm covering the third branchial arch; that these 

 filaments first appeared at the lower part of the evagination of 

 that arch; then peripheral to this point as the mound assumed a 

 wedge-shape and the wedge became compressed into a filament- 

 bearing ridge, about half a millimeter in length. Such is the 

 condition at the middle of the sixth day, at a time when the plica 

 opercularis is coextensive with the width of the neck, and when 

 the pectoral grooves and other evidences of the median migration 

 first appear in the wall below (fig. 7). 



As the zone of fusion between the under side of the opercular 

 fold and the neck moves inward from its original position at the 

 junction of the lateral and pectoral margins of each side, the 

 row of filaments is carried with it. Concurrently each row in- 

 creases its length until it reaches a maximum extent of nearly a 

 millimeter, and numbers some eight to a dozen separate filaments. 

 These are often irregularly arranged and are sometimes grouped 

 into two parallel rows. Starting with the medial end the small 

 ones with which the line begins pass abruptly into large-size 

 filaments which continue from a third to half way across the 

 line. Lateral to this medial portion there are usually gaps in 

 the line and the different members vary in height, tending how- 

 ever to become somewhat smaller as the lateral end is approached. 

 At exactly what point new filaments are added to give the line 



