176 CONK LIN. [Vol. XIII. 



. . . The term ' spiral ' refers to the fact that the curved 

 radii, if prolonged, would form a spiral about the &g^ axis." 

 Lillie ('95), commenting on this statement, says: "In the onto- 

 geny there is no twisting of the radii, but merely an inclination 

 of the axis of the dividing cell from the vertical. It seems to 

 me, therefore, that this form of cleavage would be more cor- 

 rectly termed oblique!' Kofoid ('95) has also criticised the term 

 spiral cleavage as ambiguous and misleading, and suggests as a 

 substitute alternating cleavage. It seems to me, however, that 

 the name used by Wilson is better than either of these later 

 suggestions. Lillie's only objection to the term, apparently, 

 is the fact that there is no twisting of the radii; in Crepidula, 

 however, there is in the early cleavages just such a twisting of 

 the radii as Wilson mentions, dependent upon the actual rota- 

 tion of the blastomeres after they have been formed.^ It is 

 true that in many animals with this type of cleavage an actual 

 rotation of the blastomeres and the consequent twisting of the 

 radii do not occur, but in all such cases there is at least a 

 virtual rotation. But the principal reason for preferring the 

 term spiral to the word oblique is that it has long been used 

 (Selenka, Lang, Wilson, Heymons) in one form or another to 

 designate that kind of oblique cleavage in which the divisions 

 are in the same direction in each quadrant, so that it has 

 come to have in this connection a distinctly technical meaning, 

 whereas, as commonly used, oblique cleavages may be in dif- 

 ferent directions in different quadrants, i.e., they may be bilat- 

 eral or radial or neither. The suggestion made by Kofoid 

 merely emphasizes the alternating character of the successive 

 cleavages, and this might be more satisfactorily accomplished 

 by the use of the term alternating spirals, since alternation is 

 as characteristic of orthoradial as of spiral cleavages. There 

 are cases, as we shall see a little later, in which the cleavage 

 is oblique in the same direction in each quadrant {i.e., it is 

 spiral), but in which it does not alternate with the preceding 



1 Almost all who have written on the cleavage of gasteropods (Fol, Rabl, 

 Blochmann, Heymons) describe an actual rotation of the cells. In Crepidula this 

 rotation is especially well shown in the formation of the first and second quartettes 

 and in the subsequent division of the first ; it is not marked in the formation of 

 the third quartette, but is very pronounced in the separation of the fourth. 



