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BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY. 



of the left posterior one, i. e. if we adopt Wilson's orientation instead of 

 Rabl's. In the table given above it will be noticed that the mesoderm 

 cell M is placed as a member of the basal quartet of the sixth generation. . 

 It should be remembered, however, that Rabl in a later paper ('80) 

 revised this earlier opinion as to the origin of the mesoderm cell, deriving 



CLEAVAGE OF PLANORBIS. 

 Rabl ('79). 



it this time by division from the cell M of the above table. According 

 to this the mesoderm cell in Planorbis, as in many other forms, belongs 

 to the quartet 7.2. It seems to me that the orientation of the stages of 

 Planorbis previous to the formation of the mesoderm is still an open 

 question. As I have stated in the discussion of Limax, the orientation 



