No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 429 



this form. This twist is slight, and can no longer be detected 

 when the cross comes to be mainly resolved into two isolated 

 patches of small cells. It is possible that the torsion of the 

 cross is never really lost, and that it is in some way connected 

 with the beginning of the reversed asymmetry of the animal. 

 If this were true, the reversed asymmetry of the adult would be 

 shown to be connected with the reversal of cleavage of the Q.gg. 

 The Q^g, however, passes through stages of development in 

 which it exhibits a well-nigh perfect bilateral symmetry, so that 

 it seems scarcely possible to connect directly these two phe- 

 nomena. There must, however, be some structural basis for 

 the asymmetry of the adult in the stages which exhibit such 

 marked bilateral symmetry. The effects of reversed cleavage 

 may be seen in the asymmetrical arrangement of certain cells 

 forming the cross in the first stages of gastrulation. It does 

 not seem improbable that this asymmetry persists in stages in 

 which it can no longer be observed, and that it forms the struc- 

 tural basis of the reversed asymmetry of the adult. 



The Relatio7i between Reversed Cleavage and the Direction 

 of the First Cleavage Plane. 



If we compare the four-cell stages of the eggs of Lymnaea 

 and Planorbis in respect to the relation of the first cleavage 

 plane to the median axis of the future animal, the fact will 

 become manifest that, in the two cases, this median axis is cut 

 by the first cleavage plane at a different angle. In Planorbis 

 and Physa the first cleavage plane makes with the median axis 

 a negative angle of about 45° ; while in Lymnaea it cuts this 

 axis so as to form with it a positive angle of 45°. It is obvious 

 that, while in both cases the first cleavage plane is oblique to 

 the median axis of the future animal, the first cleavage plane in 

 the sinistral form is at right angles to this plane in the dextral 

 form. As the second cleavage plane is always at right angles 

 to the first, is it not reasonable to suppose that, in the sinistral 

 gasteropods, the first cleavage furrow corresponds with the sec- 

 ond in the other forms .-• Has there not been, in the reversed 

 forms, simply a reversal of the order in which the first two 



