Some Plienoau'iia of Regeneration in Lininodriliis arui related Forms. 427 



tlie mesoblastic bands. Bergh {Lumhricus) maintains tliat tlie fir- 

 cular muscles arise from the rows of ectoderm cells outside of tlio neuro- 

 blasts i. e. froni Wilson's "nephridial" and "outer" rows. These rows 

 he terms tlie »äußere Muskelplatten«. It may be well to quote him, 

 »alle drei Zellreilien bilden eine Platte, die, am lateralen Rande der 

 Neuralplatte beginnend sich eine Strecke lateralwärts erstreckt, als eine 

 tiefere Ectodermschicht von gewöhnlichen Epidermiszellen abgelagert. 

 Im nächst abgebildeten Schnitt . . . sind die Zellen der äußeren Muskel- 

 platten bedeutend abgeplattet und sind eine gute Strecke lateralwärts 

 gewuchert ; überall bilden sie eine einfache Schicht .... Noch weiter 

 lateralwärts verbreitert und noch mehr abgeplattet sind die Elemente 

 der äußeren Muskelplatten im nächsten Querschnittsbild; sie liegen 

 hier als spindelförmig ausgezogene Zellen zwischen der Epidermis und 

 den inneren Muskelplatten, gegen die sie immer durchaus scharf ab- 

 gegrenzt sind«. He then goes into a more detailed description. 



My own observations on Tuhifex and Limnodnlus lead to the 

 conclusion that the circular muscles are regenerated from the ectoderm 

 cells in situ. Aside from the actual process of formation, one of the 

 strongest arguments in favor of the ectodermal origin of the circular 

 muscle is its position with reference to the longitudinal muscles and 

 the Order in which the two are formed. The longitudinal muscles are 

 the first to be formed. They can be distinctly seen between the neoblasts 

 and the ectoderm at a point posterior to where the Organization of the 

 neoblasts into septa is visible. The circular muscles are first visible 

 at a point nuich farther toward the anterior end in a region where 

 the mass of neoblasts is already highly organsized, approximately at 

 the level where the ventral nerve cord is being formed (Fig. 14). At 

 this point there is quite a thick band of longitudinal muscles between 

 the neoblasts and the ectoderm. Since the longitudinal muscles are 

 thus present before the circular muscles they form a wall which cuts 

 off all commuiiication between the exterior and the neoblasts, there- 

 fore in order for the circular muscles to arise from cells of mesoblastic 

 origin it would be necessary for this material to assume a position 

 between the longitudinal muscles and the ectoderm before the longi- 

 tudinal muscles were formed. This is highly improbably under the 

 circumstances and has no evidence to support it. No cells leave the mass 

 of neoblasts and assume a position exterior to the longitudinal muscles 

 before thesc are formed and no cells are at any time found between 

 the longitudinal muscles and the ectoderm. Under these conditions 

 the only possible source from which the circular nmscles can arise is 



