378 HOLMES. [Vol. XVI. 



those cells which were not separated by the last cleavage. In 

 addition, a central cleavage cavity occurs somewhat later which 

 reaches a considerable size and assumes a quadrilateral form. 

 All of these cavities disappear during the next cleavage. 



The Third Cleavage. 



The third cleavage in Planorbis is laeotropic. As in some 

 other forms, the amount of rotation is more pronounced during 

 the later stages of cell division, and the cells of the upper quar- 

 tette finally lie in the angles between the larger lower cells. 

 After the shifting has taken place the cells flatten, and there 

 results once more an almost spherical mass of cells. A central 

 cleavage cavity again makes its appearance, and again disap- 

 pears during the next cleavage. The four upper cells resulting 

 from this division, or the first quartette of ectomeres, are formed 

 of clear granular protoplasm, which gives them an appearance 

 quite distinct from the lower cells which contain the yolk. 

 While much smaller than the lower cells, or macromeres, they 

 are considerably larger than the corresponding cells in the eggs 

 of most gasteropods. The cells \a and \c meet in a polar fur- 

 row, which is inclined at a considerable angle to the polar 

 furrow at the vegetal pole. 



It seems probable, as suggested by Lillie, that Rabl has 

 made some errors in the orientation of the eggs figured in his 

 first plate, which tend to produce confusion regarding the rela- 

 tion of the two polar furrows. In the first place, Rabl's figures 

 indicate that, in the four-cell stage, the upper and lower cross 

 furrows are parallel, which is the reverse of what occurs in the 

 species here described. In case B and D meet above, as well 

 as below, we might expect that their derivatives, \b and \dy 

 would also meet in a cross furrow, whose angle with the lower 

 furrow would approximately measure the amount of rotation of 

 the micromeres. Kofoid's Fig. D ('95, p. 53) would represent 

 their relation under such a supposition. Rabl's Fig. \\A indi- 

 cates a further rotation of the upper polar furrow until it lies 

 at right angles to its original position. Fig. \2A, on the other 

 hand, shows this furrow at right angles to its position in \\A. 



