62 Charles Zeleny. 



may assume that the distal cuts do not affect the embryonic tissue 

 at the basal suture as quickly as do the more proximal cuts and 

 for this reason the tissue will have had only a small start when the 

 opposite rudimentary already has a very considerable one. The 

 latter may then restrict the further development of the bud after 

 the old stalk has fallen off. In Group IV, on the other hand, the 

 cut is so near the embryonic tissue of the suture itself that it may 

 directly injure the cells which are to give rise to the mechanism of 

 a cleaving plane or. else leave such short leverage in the distal por- 

 tion of the stalk as to compel the growing tissue to do all the work 

 in pushing off the useless portion and thus to retard its growth. It 

 is possible also that in some of the cases in Group IV the short por- 

 tion distal to the suture is not cast off, but that the tissues are 

 re-formed and thus give rise to the growing bud. However, no 

 observations were made on this last point and it is merely put 

 down as a possibility in view of some of the later experiments on 

 Pomatoceros (p. jd). 



In the above -paragraph an attempt has been made to show how the 

 embryonic tissues at the basal suture in Group I and in Group IV 

 may develop up to the stage of a rudimentary operculum less rapidly 

 than those of Groups II and III . 



Second Factor. Admitting this greater development of the bud 

 of the old functional side in Groups II and III than in Groups I 

 and IV, and further assuming the uniform development of the 

 bud of the old rudimentary side in all four groups, we are led to the 

 consideration that if one of the opercula when well developed com- 

 pels the other to stay in a rudimentary stage such a cause may act 

 in Groups I and IV and not in Groups II and III. Therefore, in 

 the former cases, the result of the simultaneous operation on both 

 opercula is a reversal of the original condition, while in the latter it 

 ts the production of two ^^ functional opercula. 



f. Body Cut in Two. Regeneration of Opercula at the Anterior 

 End. When the body is cut in two in the thoracic region two 

 opercula and groups of branchiae are regenerated on the two sides 

 of the median line but the opercula instead of being differen- 

 tiated into a large one and a small one are both of the large func- 

 tional type. We must assume in this case that since both had an 

 equal start in development the retarding influence of the one upon 

 the other which occurs in other cases did not occur here. 



The newly developed opercula were in some cases exact dupli- 



