4o6 HOLMES. [Vol. XVI. 



cells can be given. The group of cells in the b quadrant of the 

 second quartette is shorter and broader than the other groups. 

 This is doubtless due to the forward rotation of the apical pole 

 of the Qgg, which would exert a vertical pressure on this group 

 of cells. The slower cleavage in this group may be also due, 

 in a certain degree, to the same cause, though it is probably 

 correlated with the fate of these cells. The portion of this 

 quadrant which does not go into the prototroch forms only an 

 exceedingly small portion of the body of the embryo, being 

 used, as far as could be determined, to form a part of the 

 stomodaeum. 



The early cleavages of the second quartette in Planorbis are 

 very similar to those of other gasteropods. Kofoid has traced 

 the cleavage of this quartette in Limax to a stage in which 

 there are four cells in each quadrant, and these cells agree 

 almost exactly in relative size and arrangement with those in 

 Planorbis at the same stage. Blochmann ('82) figures a stage 

 in which there are seven cells of this quartette in each quad- 

 rant, and their arrangement is very similar to that found in 

 Planorbis. Heymons has traced the lineage of this quartette 

 to a stage in which there are eleven cells in each quadrant, but 

 he does not describe the direction of the cleavage of the tip 

 cells. Conklin has followed the cell lineage of the second quar- 

 tette to a stage in which each quadrant contains eleven cells, 

 and has traced the cleavage of the tip cells somewhat beyond 

 this point. The agreement of the direction of the divisions of 

 this quartette in Planorbis with those in the above forms is quite 

 close, but there are some minor differences. In general, we 

 may say that the cleavages are more nearly radial than in the 

 two last forms. For instance, the cleavages of 20'''^''^ in Crepi- 

 dula and 2rt'"^'^ in Umbrella are nearly horizontal, while in Pla- 

 norbis they are nearly radial. All the cells lying between the 

 tip cell and the lowest cell of the group have, at a stage when 

 there are eleven cells in each quadrant, exactly the same lineage 

 in Umbrella, Crepidula, and Planorbis. In the first two forms 

 the cleavage of the tip cell has occurred, while the lowest cell, 

 2a^''', 2^^ "^ etc., remains undivided ; in Planorbis the lowest cell 

 has divided, while the tip cell, except in the anterior quadrant. 



