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1885) by which the first and second cleavages became interchanged. 

 Accordingly he has treated the cleavage of Planorbis as if its first 

 division corresponded to the second of Crepidula and other dextral 

 forms, the first cleavage in the former separating the macromere AD 

 from BC, whereas in the latter the first cleavage separates the macro- 

 mere AB from CD (see Fig. 4). Such an interpretation is fraught 

 with serious difficulties, for in order to preserve the "dextral" 

 nomenclature of the cells it is necessary to assume that the parts 

 which arise from the cell A in dextral forms come from the cell C in 

 sinistral ones, or, what is still more improbable, considering the fact 

 that both dextral and sinistral forms may occur within the limits of 

 a single species, that the suppressed organs of the left side in a dextral 

 form become the developed organs of a sinistral animal while the 

 organs of the right side are here suppressed. On the other hand, 

 Crampton's (1894) view (as expressed by his labelling of the cleav- 

 age cells of Physa Fig. 4L) that the cleavage in this case is the 

 inverse or mirrored image of the usual form of cleavage, accords 

 with all the facts and furnishes besides an explanation of the inverse 

 symmetry of the adult. 



If the inverse symmetry of the adult can be traced back in the 

 development to inverse embryos, to inverse cleavage, and finally to 

 the unsegmented egg itself, how may we conceive of these dextral 

 and sinistral characters as existing or arising in the egg? So far as 

 I know, Rabl (1900) is the only one who has attempted to answer 

 this question. Rabl calls attention to the fact that in Limax, accord- 

 ing to Mark (1881), there are spiral asters the rays of which are 

 "right wound", while in Kostanecki and Wierzejski's (1896) figures 

 of Physa the astral rays are "left wound", Fig. 2. However, it is 

 doubtful whether such a constant difference as this exists. In the case 

 of the outer aster of the first muturation spindle Mark says, p. 194, 

 "The curvature in the cases I have seen (remembering that the ob- 

 server is looking upon the animal pole) is such as would be produced 

 by the peripheral ends of the fibres being moved in the direction in 

 which the hands of a clock advance, while the centre remained fixed". 

 But with respect to the inner aster of the second maturation spindle 

 he says, p. 209, "Both the extent of the rays and the degree of 

 their curvature are subject to great variation in different eggs. . . . 

 Nor does it appear that the direction of the spiral is constant, for 

 while my earlier observations chanced on cases in which the course 

 was left-handed, later studies taught that the reverse was not 

 uncommon, though probably of less frequent occurance." Furthermore 



