100 Charles Zeleriy. 



(2.) Relations of time of moult and size of regenerated structure similar to those 

 of Gelasimus. 

 5. Apomatus. Serpulid. Opercula OpRor l and ORLor r. (See Fig. 23D.) 



Operation. Removal of two branchial circlets + opercula at basal suture. 

 Uniform in all experiments. 



(i) In one set of experiments body of animal is kept entire. 



(2) In other set of experiments body of animal back of second or fourth thoracic 

 segment is removed. 

 Result: 



Rate of differentiation of new ^^.^ Rate of differentiation of new opercula 

 opercula in (i) ^^"^ in (2). 



because 



Retarding influence of whole ^--.^ Retarding influence of anterior two or 

 body (i) -"""^ four thoracic segments (2). 



The superficiality of the attempted explanation of the data is 

 very apparent, but it is from this point of view that the analysis of 

 the problems must be attacked. That the factors which it is 

 attempted to isolate are real factors in normal development is 

 shown by the physiological reversal of the opercula which takes 

 place in the ontogeny of Hydroides, a process in all ways similar 

 to the reversal after artificial section of the stalk of the functional 

 operculum. From such a point of view the present analysis 

 may be of value as affording a basis for further experimentation 

 on the isolation of the factors involved in regulation. 



Hull Zoological Laboratory, 



University of Chicago, 



May 17, 1904. 



