No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 397 



vation of the basal cells of the cross was likewise incorrect. 

 Interpreting Blochmann's figures in the light of what is known 

 to be the rule in other forms, it is evident that the cell lineage 

 of the cross in Neritina, at the stage when there are three cells 

 in each arm, corresponds exactly to that of Umbrella, Crepidula, 

 and Planorbis. In the lengthening of the posterior arm of the 

 cross by the addition of another cell, there is a further point of 

 agreement with the history of the cross in the last two forms. 

 In Crepidula both the basal and the tip cells of the posterior 

 arm of the cross divide radially at about the same time, so that 

 this arm comes to have one cell more, instead of one less than 

 the other three. Two of the four cells in this arm, therefore, 

 belong to the first quartette and two to the second. Whether 

 three of the four cells of the posterior arm in Neritina belong 

 to the first quartette, or whether there are two cells each of 

 the first and second quartette, is uncertain. Blochmann says 

 nothing about the derivation of this additional cell. Should we 

 argue from analogy with Crepidula, we should be led to accept 

 Conklin's scheme of the probable derivation of this cell, and 

 derive the two posterior cells from the second quartette. If, 

 on the other hand, we should draw our conclusion from a com- 

 parison with Planorbis, we should infer that the three anterior 

 cells belonged to the first quartette, while only the tip cell 

 belonged to the second. In Planorbis the four cells in each 

 arm have the same derivation and are produced at nearly the 

 same time. In both Neritina and Crepidula, owing to a delay 

 in the division of the basal cell, the posterior arm contains at 

 first but two cells, while each of the other arms contains three ; 

 this stage is followed by a stage in which the posterior arm 

 contains four cells, while the number in the other arms remains 

 the same. The tip cell in the posterior arm in Crepidula is 

 larger than those of the other arms and divides first, and in a 

 different direction from the others ; that is, radially instead of 

 transversely. The divisions of the cross cells in Planorbis are 

 such that they preserve the radial symmetry of the cross much 

 longer than in Crepidula and Neritina. There is little differ- 

 ence, either in the time or direction of the cleavages, in the dif- 

 ferent arms until after the period in which each arm is composed 



