Reversal of Cleavage in a Sinistral Gasteropod. 169 



their division here is in a /e/^handed spiral. In Limn^ea and other 

 forms, they divide to the right. 



A twenty-four-cell stage obtains by the division at about the 

 same time of the second group of micromeres and the macromeres. 

 A third group of micromeres arises spirally to the left, reversing 

 the condition in Limna^a. And moreover the cells a^, IP, etc., divide 

 at riffht-angles to the direction in other forms. 



There are one or two most interesting questions which arise in 

 view of the foregoing facts. I shall not attempt to discuss these, 

 contenting myself with merely pointing out their interest. First, 

 is there any correlation between the sinistral shell of the adult 

 Physa and the unique method of cleavage? The answer to this 

 will depend on further investigations on the embryology of sinis- 

 tral forms. Heretofore, we have had slight evidence of reversed 

 cleavage. Planorbis, as figured by Rabl (Morph. Jahrbuch, Band 

 5, 1879), is certainly reversed in some stages ; whether this obtains 

 throughout is uncertain from the figures. The only other case is 

 that of Janthina (Haddon, Quarterly Journ. Micro. Sc, vol. xxii, 

 N. S., 1882); there is a figure of an eight-cell stage, which appears 

 reversed. Janthina has an ordinary dextral shell. 



Another important question is that relating to the origin of the 

 mesoderm. In all the spiral forms known cytogenetically, the 

 mesoderm arises from D, as one of the fourth group of micromeres, 

 which is given off in a left-handed spiral. And as D is on the left 

 side, d* comes to lie at the median posteinor part of the embryo. Now 

 in Physa, D is on the right, and d^ must arise in a rty/i^handed 

 spiral ; so that, while it occupies the same position as it does in 

 other forms, still in Ph^^sa, it is derived from the opposite side of 

 the Q^g. I shall not attempt to discuss whether this argues for 

 isotropy or preorganization, but will reserve full treatment for the 

 future. 



Columbia College, 1894. 



