No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 435 



these forms is correlated with the bilaterality of the future 

 animal. May not the delayed appearance of bilaterality be due 

 to the circumstance that the oblique direction of the first cleav- 

 age started the divisions in a spiral direction, which is only 

 overcome later by the tendency to bilateral cleavage .'' 



It is worthy of note, in this connection, that in many eggs 

 the first cleavage plane is oblique to the axis of elongation. 

 This is the case, according to Conklin, in Urosalpinx, and, 

 according to Fol ('75), the first cleavage furrow in Cymbulia 

 is oblique to the line connecting the two attraction spheres. 

 Among the Rotifera, Tessin ('86) found that in Eosphora the 

 first cleavage furrow is oblique to the long axis of the o.^^ and, 

 therefore, to the median axis of the animal. In Callidina, 

 according to Zelinka ('9l), the first cleavage plane is oblique 

 to the long axis of the Q^g, but by a shifting of the cells it 

 comes, finally, to be transverse. And in Asplanchna, Jennings 

 found that the first cleavage amphiaster "lies at first somewhat 

 oblique to the longitudinal axis of the &gg, but before cleavage 

 takes place the spindle swings into coincidence with it." The 

 nucleus in the larger of the two cells subsequently rotates to 

 the right, so that a line joining the two nuclei would cut the 

 first cleavage plane at an oblique angle. The possibility sug- 

 gests itself that the rotation of this nucleus may be due to the 

 same circumstances that caused the oblique direction of spindle 

 before cleavage. The behavior of the ^gg of Asplanchna would 

 seem to indicate that it tends to divide obliquely, as in Callidina 

 and Eosphora, but that this tendency is overcome by the tend- 

 ency to divide at right angles to its longest diameter, and only 

 manifests itself at the beginning and at the end of cleavage. 

 The first cleavage of Asplanchna recalls that of Crepidula in 

 that the phenomenon of nuclear rotation is manifested after 

 the completion of division. It is quite evident that among 

 these rotifers other factors besides the shape of -the Qg% influ- 

 ence the direction of the first cleavage. According to Hert- 

 wig's law, it would be expected that the first cleavage plane 

 would uniformly lie at right angles to the long axis of the tgg. 

 The fact that the first cleavage is oblique to this axis in Calli- 

 dina and Eosphora, and manifests a tendency to obliquity in 



