434 HOLMES. [Vol. XVI. 



other at a negative, angle. We are naturally led from this sup- 

 position to the view that spiral cleavage, in general, may be due 

 to the oblique direction of the first cleavage in relation to a 

 preformed longitudinal axis of the ovum. The possibility is 

 open, on the other hand, that the direction of the first cleavage 

 is not predetermined in the ovum, and that the median axis is 

 determined only during cleavage. The evidence, however, is 

 apparently against such a view, although it is still far from 

 complete. The fact that the isolated blastomeres of the gas- 

 teropod ^g^ exhibit the phenomenon of "partial development" 

 (Crampton, '96) in a marked degree affords a very strong reason 

 for regarding the median axis of the embryo as determined at 

 the time of the first division. It does not necessarily follow 

 that this conclusion is to be extended to all forms with de- 

 terminate spiral cleavage ; yet Crampton's experiments may 

 rightly be held, I think, as affording no little support to this 

 generalization. 



If the first division were always of a spiral character, as it is 

 in Crepidula, the second cleavage would, as a consequence, be 

 also more or less oblique, and the spiral character of the suc- 

 ceeding cleavages may be regarded as simply a consequence 

 of Sach's law, that the direction of every cell division tends 

 to be at right angles to the preceding division. The second 

 division in forms with determinate spiral cleavage is, I believe, 

 always a spiral one. In Planorbis, as in Physa (Crampton, '94), 

 Limax (Kofoid, '95), Crepidula (Conklin, '97), and Amphitrite 

 (Mead, '97), the spiral character of this cleavage is manifested 

 by the inclination of the spindles before a very marked constric- 

 tion of the cytoplasm takes place, and cannot, therefore, be 

 regarded as a consequence of the shifting of the blastomeres. 

 It seems not improbable that the spiral character of the second 

 cleavage is a consequence of a spiral tendency of the first cleav- 

 age — that the agencies that cause the rotation of the nuclei 

 and spheres in Crepidula after the first division are present in 

 other eggs also, although they produce no visible effect until 

 the second cleavage. In both annelids and mollusks, spiral 

 cleavage is soon superseded by cleavage of the bilateral type, 

 and there appears no reason to doubt that bilateral cleavage in 



