Compensatory Regulation. 



57 



more and more the character of the former functional operculum. 

 After 15 to 20 days the development is complete and we have a 

 complete reversal of the opercula. The former functional oper- 

 culum is now the rudimentary and the former rudimentary has 

 become the functional. The resulting arrangement is the exact 

 reciprocal of the former one. 



The case just briefly outlined will now be taken up in more 

 detail. Before the stalk of the functional operculum is cut it is 

 almost impossible to pull it off^ from the animal. The basal suture 

 resists breaking as well as the solid material of the stalk. The 

 hardest kind of a pull that can be 

 given with a pair of forceps is not suffi- 

 cient to dislodge the organ. Inside of 

 a few days after the operation, how- 

 ever, the opercular stalk as we have 

 seen comes off^ of its own accord, so 

 that great changes must be assumed 

 to have taken place in the region. 

 The time at which the stalk drops off 

 varies greatly. In one case it had 

 not come off 5 days after the operation 

 and in another after 6 days it was still 

 attached. Very soon after the stalk 

 has dropped off a bud appears at 

 the top of the stump and this stead- 

 ily increases in size until the typical 

 form of a rudimentary operculum is 

 reached. (Fig- 19 a, a-f.) It seems 

 very probable that the breaking off of tion at the distal end of a functional 

 the remanent of the stalk is induced ^taik which had been cut two-thirds of 



1 1 ■ 1 • 1 1 • I the distance from the base to the termi- 



by histological changes in the suture , o^, , - ■• • , 



■1 . 1 • 1 1 ''^^ '-'^P- ^"^ °'" rudimentary is also 



region which are attendant upon the shown. 



beginning of the development of the 



new bud. Evidence in favor of this view will be given later in 



another place. 



Except in one case no differentiation of tissues took place at the 

 cut end of the functional stalk. In this case there was an expan- 

 sion on each of the two sides of the stalk near its tip. The stalk 

 did not drop off for 5 days after the operation. The changes are 

 represented in Fig. 20. 



Fig. 20. 

 Opercula of Hydroides dianthus, five 

 days after transverse section of stalk 

 ( X 26). Exceptional case. Differentia- 



