42 CONK LIN. [Vol. XIII. 



tion, and it must therefore have been predetermined during, and 

 perhaps before, the first cleavage. 



I have not been able thus far to discover by what means or 

 in what manner this movement is predetermined. In Crepid- 

 ula the first spindle does not seem to indicate any such rota- 

 tion, though it is exceedingly suggestive to note that Warneck 

 (•50) in the case of Limax and Fol ('75) in Cymbulia found 

 that the first cleavage was oblique to the axis of elongation of 

 the ^g^. Kofoid ('95) however, in his recent careful work on 

 Limax, found no evidence in favor of Warneck's account. In 

 some cases in which the first cleavage is very unequal, as e.g., 

 in Urosalpinx, the plane of the first cleavage is oblique to the 

 axis of elongation, and it may be that it is also oblique to the 

 polar axis of the t^%. 



But however the direction of these movements may be pre- 

 determined, the fact that they are predetermined, at least 

 during the period of the first cleavage, is a profoundly signifi- 

 cant one, indicating as it does that the first cleavage of the egg 

 belongs to a series of ''spiral" cleavages which for at least nine 

 successive generations of cells are alternately dexiotropic and 

 laeotropic. 



Strictly speaking the first cleavage could scarcely be called 

 a spiral one, since there is but a single spindle which inter- 

 sects the chief axis of the &gg ; and besides there is no defi- 

 nite cross axis to which the direction of this spindle can be 

 referred. It is certain, however, that the dexiotropic turning of 

 the nuclei and protoplasmic areas after the first cleavage is, on 

 the one hand, causally related to their laeotropic turning during 

 the second cleavage, and on the other hand it seems to be pre- 

 determined at least as early as the preceding cleavage. It is 

 therefore highly probable that the first cleavage belongs in the 

 same category with the succeeding spiral cleavages, though per- 

 haps it would be more exact and less paradoxical to speak of it 

 as prospectively spiral and dexiotropic. 



These so-called " spiral " cleavages are always radially sym- 

 metrical.i A glance at Fig. 6 or 7 will show that the two 

 blastomeres are not mirrored representatives of each other, i.e., 



1 This subject is treated at length in the concluding section of this paper. 



