234 C. W. Hahn. 



asexual reproduction, since the processes leading up to them have 

 been observed in regeneration artificially induced. In view of 

 these facts it is improbable, as has been supposed by several 

 investigators, that double Metridia are a stage in a process of repro- 

 duction by longitudinal fission. The older viev^ that they are 

 genuine monstrosities seems better supported, but not the view 

 that they are due to coalescence, as was once thought to be the 

 case. 



From long strips cut from the margin of the foot-disc and 

 including a half or more of its circumference, polyps with two or 

 three distinct oral discs have several times been obtained in these 

 experiments. This result throws still further light on the origin 

 of double monsters. 



SUMMARY. 



The dimorphism which occurs in Metridium is due not to 

 alternative inheritance of the diglyphic and monoglyphic condi- 

 tions, but to the frequent occurrence of asexual reproduction. 

 This takes place spontaneously by basal fragmentation and may 

 readily be induced by cutting off pieces of the foot-disc. Whether 

 a particular fragment produces a monoglyphic or a diglyphic 

 polyp depends, not on the monoglyphic or diglyphic condition 

 of the parent polyp, but upon whether the fragment does or does 

 not contain some portion of a directive system, for a directive 

 system is regularly produced in the regenerated portion of the 

 young polyp. Accordingly, if the portion derived from the parent 

 already contained a directive system, the young polyp will have 

 two such systems and will be diglyphic. But if the parental 

 fragment contained no directives, the young polyp will have only 

 one directive system, that produced in regeneration, and will be 

 monoglyphic. 



Not only the dimorphism of Metridium, but also its prevailing 

 asymmetry and extreme variability in number and arrangement 

 of mesenteries can be explained by its method of development 

 in asexual reproduction. Triglyphic polyps and those with two 

 or more oral discs or with double or branched oesophagus or 

 devoid of siphonoglyphs are abnormalities due probably to 

 regeneration from fragments of unusual size or shape, as compared 

 with the fragments normally produced in spontaneous basal 

 fragmentation. 



