No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 443 



not a demonstration — through the facts of reversal of cleav- 

 age. It can readily be seen that, by virtue of reversed cleavage, 

 the relative positions of certain cells become different from those 

 they would occupy if the cleavage were of the normal or unre- 

 versed type. For example, the position of the cells of the third 

 quartette in each quadrant is, in the dextral forms, to the right, 

 and, in the reversed forms, to the left of the cells of the second 

 quartette. Similarly, the trochoblasts lie to the right of the 

 apical cells in the dextral forms, and to the left of these cells 

 in the sinistral forms. In the dextral forms the cells in the 

 two anterior quadrants of the third quartette are 3^; and ^by 

 and in the posterior quadrants 3^ and 3^. In the reversed 

 forms 3^ and 3^ are anterior, and 3^ and 3^ posterior. In the 

 dextral forms the trochoblasts are Irt^ i^^ anterior, and \c^, id^, 

 posterior ; while in the reversed forms the anterior trocho- 

 blasts are lU^, id^, the posterior, la", id"". In both reversed and 

 unreversed forms the corresponding arms of the cross are 

 derived from the same quadrants. 



The anterior trochoblasts in both Crepidula and Planorbis go 

 to form the prototroch, and the posterior ones go into the head 

 vesicle. The cells which have a similar position in the two 

 forms have the same fate, although they have a different origin. 

 The cells which go into the prototroch in Crepidula are id^ and 

 ib^y while in Planorbis the cells which have this fate are ib^ and 

 I^^ Conversely, cells of the same cell origin have different 

 fates, viz., id^ goes into the prototroch in Crepidula, while in 

 Planorbis it forms a part of the head vesicle. Although the 

 cell ib^ goes into the prototroch in both forms, it forms a part 

 of the right side of this structure in Crepidula and a part of the 

 left side in Planorbis. It certainly appears that, in a certain 

 sense, the fate of a cell is a function of its position. It has 

 been remarked by Driesch that, if the blastomeres of an Qgg 

 could be shifted about at will, their development would take 

 place in accordance with their relative positions. While in 

 reversed cleavage nature has performed an experiment for us 

 in the shifting of blastomeres, and while the results show that 

 the fate of the cells is in accordance with their position, and 

 not their genealogy, the experiment differs considerably from 



