Cornpensatory Regiilatiofi. 77 



separated nerve cords and other cases, for which reference must be 

 made to a future paper, indicate such a control. (See p. 62.) 



The study ot the ontogeny of the opercula in Hydroides has 

 cleared up the subject greatly. It is seen that the experimentally 

 obtained reversal is merely an expression ot a normal develop- 

 mental process. The primary functional operculum develops as 

 an asymmetrically placed (left) structure and by virtue of its early 

 development holds in check the embryonic opercular tissue of the 

 opposite side. Later the primary functional operculum drops off, 

 the former rudimentary tissue is no longer retarded, and, therefore, 

 develops to a functional stage when it in turn restricts the now 

 developing bud of the other side. Evidently the expermiental 

 reversal is merely an expression of this same process, the artificial 

 removal of the functional operculum by a transverse cut causing 

 the removal of the retardation effects on the opposite bud, and at 

 the same time hastening the formation of the embryonic tissue at 

 the basal suture of its own side, therefore leading to the dropping 

 off of the remanent of its stalk above the suture and leaving room 

 for the new rudimentary to develop as far as the now rapidly 

 developing new functional of the opposite side will allow it. The 

 fact that there is such a retardation effect of the larger organ on the 

 smaller is further indicated by the fact that after transverse section 

 of the body the two opercula which develop are both large and 

 resemble the functional in general character though differing from 

 it in particulars. The same thing is further indicated by the 

 possibility of an explanation on this basis of the complicated results 

 obtained when both opercula are cut off. It has been shown 

 (p. 61) that assuming such an interaction between the opercula 

 as here given we can explain the development of two functionals 

 in one group and the reversal of the opercula in the other by the 

 earlier dropping off of the functional stalk in the former group as 

 compared with the latter, giving the bud of the functional side in 

 the first case an equal chance in the competition with the opposite 

 one, which chance it does not possess in the second case. 



IV. THE RATE OF DIFFERENTIATION DURING REGENERATION OF 

 THE OPERCULA IN THE SERPULID, APOMATUS. 



The effect of the size of the piece upon the rate of regeneration 

 of the opercula may be very conveniently studied in this species 



