168 Reversal of Cleavage in a Sinistral Gasterojjod. 



lin, Zool. Anz., No. 391, 1892), and others. To the facts of this 

 nature, I add, as my results, that the cleavage of Physa is typically 

 spiral, but totally reversed. This is not abnormal, for all the series 

 obtained developed reversedly, while Limna3a eggs, laid in the same 

 jars, proceeded in the regular direct manner. 



The eggs are smaller in Limnrea than in Physa. Segmentation 

 begins about five hours after deposition, and about two hours after 

 the extrusion of the polar globules. As usual, the first cleavage- 

 plane is meridional, dividing the egg into equal parts, anterior and 

 posterior. The second plane, also meridional, divides each of these, 

 forming four nearly equal blastomeres. A, B, C, D (Figs. 1 and 2), 

 D being slightly larger than the others. The first evidence of 

 variation in Ph3^sa appears at a late 2-cell stage. Here the spindles 

 for the next cleavage, instead of being inclined from left below to 

 right above, in side view, as they are in Limnaea, are inclined from 

 right below to left above. This is a total reversion, which in the 

 completed 4-cell stage (Fig. 2), brings about the reversion of the 

 "cross-furrows." The appearance of these in Limncea and other 

 forms cited is that shown in Fig. 1. 



And here it is plainly seen that the crossing of the upper and 

 lower cross-furrows is an expression of a spiral cleavage ; for the 

 cells B and D being in contact at the lower pole, while A and C 

 are in contact at the upper, it follows that A and C are at a higher 

 level. And as A is separated off from B, and G from D, we have 

 a spiral cleavage, to the left in Limmea, and in Physa to the right. 

 A very noticeable feature at this stage is the enormous blastocoel. 



The third cleavage plane is equatorial, and separates the first 

 group of micromeres, a\ &', c\ d^, from the macromeres, A, B, C, D 

 (Fig. 3). These smaller cells alternate with the macromeres, show- 

 ing the spiral nature of the division. But it will be noticed that 

 they are derived in a Ze/if-handed spiral, the reverse of the case in 

 Limnffia ; and that this direction is opposite to that of the spiral of 

 the last division, as to be expected in a normal cleavage. 



The twelve-cell stage occurs next. A second group of micro- 

 meres, a^ b^, c^ d^ (Fig. 4), is given off from the macromeres in a 

 right-handed spiral, this being opposite to their origin in other 

 forms ; and also alternating with the spiral of the third cleavage. 



Then the cells of the first group of micromeres divide unequally 

 (Fig. 5, a'-\ a^-~, //-^ 6'-^ etc.). Here again the reversal holds, as 



