430 HOLMES. [Vol. XVI. 



cleavage planes make their appearance ? And is not this the 

 circumstance that determines the different direction of spiral 

 cleavage in the reversed gasteropods ? 



It has been discovered by Conklin that the first cleavage in 

 Crepidula is prospectively spiral and dexiotropic, as indicated 

 by the rotation of the nuclei in the two-cell stage from left to 

 right. I have not convinced myself that there is an opposite 

 rotation of the nuclei in the two-cell stage of Planorbis, but the 

 second cleavage is clearly dexiotropic even in its first stages. 

 There is doubtless some structural basis for the different char- 

 acter of the second cleavage in Planorbis in the two-cell stage. 

 The second cleavage of Crepidula is laeotropic, and that of 

 Planorbis dexiotropic ; and it seems probable that this difference 

 is connected with the different relations of the first cleavage to 

 the future longitudinal axis of the animal. If we suppose that 

 in Crepidula a preformed longitudinal axis is cut obliquely by 

 the first cleavage plane, it may help us to account for the rota- 

 tion of the nuclei in the two-cell stage, and consequently the 

 laeotropic second cleavage. As this axis in the sinistral forms 

 would be cut at the opposite angle, we may have, in this cir- 

 cumstance, an explanation of the different direction of the spiral 

 cleavage that is manifested in the second, if not somehow in 

 the first division of the ovum. Certain it is that, if the longi- 

 tudinal axis of the embryo is in any sense preformed in the Q.g^, 

 it is cut at planes approximately at right angles to each other 

 in the dextral and sinistral forms. An oblique cleavage in 

 relation to the bilateral organization of the Q.gg might cause a 

 certain amount of torsion that would manifest itself either at 

 the end of that division or at the beginning of the next. Given 

 two eggs in which this bilateral organization is cut at opposite 

 angles by the first cleavage plane, it is probable that the torsion 

 in the two cases would take place in opposite directions. 



General Considerations on Spiral Cleavage. 



If we glance over the literature on spiral cleavage, we shall 

 find that, in different forms, the first cleavage is said to stand 

 in different relations to the future longitudinal axis of the ani- 



