176 E. H. HARPER. 



narrowing of the intestine to form the oesophagus and a dilata- 

 tion to form the crop. The histological features are not consid- 

 ered here. The same process occurs in regeneration from a cut 

 through the intestine. 



3. Inhibition of the zone of fission under the influence of a 

 regenerating region anterior to the fission plane. 



A striking instance of regulation is seen in the effect produced 

 upon a zone of fission by section anterior to it. Under certain 

 conditions the formation of a regenerating region will inhibit the 

 process of self-division and cause the disappearance of the zone 

 of fission. This involves the rearrangement of tissues and a 

 process of redifferentiation in a region at a distance from the cut 

 surface. If the animal is severed in front of and near to a zone 

 of fission which is in an early stage of development before seg- 

 mentation has taken place, the constriction and the accumulated 

 embryonic tissue disappear and the zone is completely rediffer- 

 entiated (Fig. I, a-c]. 



The nearer the regenerating region is to the zone of fission the 

 greater is the tendency for the latter to disappear, but the in- 

 fluence may be exerted at a considerable distance. The follow- 

 ing table gives the results in 48 cases : 



Number of Segments Between the Cut Number of Experi- Disappearance of Zone of Fission 

 Surface and the Zone of Fission. ments. in. 



1-7 28 60 per cent. 



8-15 20 15 per cent. 



In all the cases included in the above table the zone of fission 

 was in an early stage of development, but not all were at exactly 

 the same stage. If only specimens were taken for experiment in 

 which the zone was just beginning to form, no doubt a much 

 larger percentage of cases of disappearance would be obtained 

 both when the regenerating region was near and when it was far 

 away. 



The writer was informed by Professor Frank Smith of an ob- 

 servation upon a nearly related form, Pristina sp., to the effect 

 that the zone of fission may disappear in animals kept in the 

 laboratory, as a result of insufficient food or other unfavorable 

 conditions. It is realized that a control experiment would be 

 desirable, to show what percentage of cases of disappearance of 



