426 HOLMES. [Vol. XVI. 



in many other gasteropods, even in the least degree. In the 

 species of Planorbis studied by Rabl the blastopore is said to 

 become the mouth. Whether this difference is merely a spe- 

 cific one, or whether Rabl failed to observe the blastopore dur- 

 ing the short time it is closed, is uncertain. 



Part II. General Considerations. 

 Reversal of Cleavage and Reversed Asymmetry. 



The fact that in certain gasteropods, with sinistral or reversed 

 shells, the cleavage is also reversed, was first pointed out by 

 Mr. Crampton ('94). Crampton studied the cleavage of two 

 closely related genera of fresh-water pulmonates, Physa and 

 Lymnaea, the former of which has a sinistral shell, while in the 

 latter the shell is of the normal or dextral type. The cleavage 

 of Physa was found to agree, point for point, up to the stage at 

 which the primary mesoblast is formed, with that of Lymnaea ; 

 but with this exception, that the direction of every cleavage is 

 reversed. The reversal was observed in the cleavage which led 

 to the four-cell stage, and was shown to give rise to a different 

 position of the cross furrow in the two forms. Crampton also 

 points out that Planorbis, according to Rabl's paper, affords 

 another instance of reversed cleavage. The possibility of a 

 causal connection between reversed cleavage and a reversal of 

 the shell was pointed out, but further discussion of the subject 

 was not attempted. Haddon's figure of the eight-cell stage of 

 Janthina ('82) apparently shows, as Crampton observes, that the 

 cleavage of this form is reversed. This would form the only 

 exception to the rule that the cleavage is dexiotropic in all unre- 

 versed forms. It seems not unlikely, however, that Haddon's 

 figure is misleading on this point. 



The cleavage of another species of Physa, P . fontinalist was 

 found to be reversed by Wierzejski, and Brooks ('79) figures a 

 four-cell stage of Planorbis parvus, which, according to his 

 statement concerning the origin of the two upper cells, affords 

 another instance of reversal of cleavage. There are, therefore, 

 two species of Physa and three of Planorbis in which the cleav- 



