A STUDY OF METAMERISM. 431 



relatively few leeches examined, although the number was 

 sufficient to show that the modification involving additional 

 half-annuli, if it occurs, must be rare. 



VIII. Summary. 



In the papers of Cori (13) and myself (32) the same ex- 

 planation was oflTered as to the origin of the simpler cases of 

 abnormal unions. Cori said, '' Nun kann es sich ereigen, dass 

 in dereinen Korperhalftewahrendder Entwickelungsperiode ein 

 Ursegment mehr gebildet wird, dem auf der Gegenseite keiu 

 Ursegment entspricht. Auf diese Weise wird die Bildung jener 

 Falle von Schaltsegmenten verstaudlich, wie sie im vorherge- 

 henden anatomisch beschrieben worden. BetrefFend das Ver- 

 haltnis eines Schaltsegmentes zu den iibrigen Metameren des 

 Korpers sind zwei Moglichkeiteu vorhanden. Es kann sich ein 

 Schaltsegment vollstandig ausbilden, und kann von den audereu 

 Segmenten abgegrenzt bleiben oder es geht eine Verbindung 

 mit dem vor oder nach folgenden Metamer ein." 



My own statement was very similar : — " We know that in 

 the embryo the metameres are laid down right and left of the 

 middle line of the body in blocks of mesodermal tissue; that 

 under normal conditions these hollow blocks come to lie 

 exactly opposite (right and left of) one another, so that the 

 opposite pairs unite across the median dorsal and ventral lines. 

 If we conceive that the blocks are slightly displaced on one 

 side, or that two consecutive blocks of one side are smaller 

 than two of the opposite side, we may have, as a necessary 

 mechanical result of the relative position of the block, a split 

 metamere.^^ To account for the spiral arrangement I said, 

 and I am still of the opinion that this is the true explanation, 

 '' If we imagine one of the mesodermic blocks of one side to 

 be larger either above or below (but not both above and below), 

 so that above (let us say) it opens into two body-cavities of the 

 opposite side, while below it opens into but one, then we have 

 produced the conditions necessary to start the spiral. Each 

 of the consecutive blocks on the same side as the supposed 

 larger block will open below into its proper opposite block, but 



