No. I.] THE EMBRYOLOGY OF CREPIDULA. 57 



at the twelve-cell stage, and probably in Neritina. In Planor- 

 bis, however, according to Rabl's interpretation, the cross fur- 

 row of the animal pole is not restored to the position of the 

 four-cell stage, but is turned 90° from it (see his Taf. XXXII, 

 Figs. 10 A, II A). To accomplish this it is necessary for 

 each of the cells of the apical quartette to be shifted 90° to the 

 left, and thus completely out of their own quadrants over upon 

 the adjoining quadrants. It seems very probable that Rabl is 

 in error in this matter, and that in Planorbis, as in the other 

 forms, the division of the generations results in the restoration 

 of the cross furrows to the conditions of the four-cell stage." 

 It is possible that in Planorbis, as in Crepidula, there is no part 

 of the original polar furrow between the cells of the apical 

 quartette, and that the pressure surface, formed later, may lie 

 in any direction with reference to the real polar furrow. 



2. Formatioji of the Secojid Quartette of Micromeres. Figs. 

 14-16, Diagram J (/. 60). 



When the first four micromeres have taken a position alter- 

 nating with the macromeres, the nuclei of the latter again 

 divide, as shown in Fig. 14. All the nuclei divide at nearly 

 the same time, as in the preceding cleavage, but the spindles 

 do not lie radially as before, but run transversely or tangentially 

 in each macromere. One end of each spindle lies on the mid 

 line of each macromere, the other end lies to the left, very near 

 the furrow, between contiguous spheres ; the former is at a 

 lower level than the latter, and hence the spindles are arranged 

 in a left wound or anti-clockwise direction. Again, considering 

 the deeper or central end of each spindle as fixed and the other 

 as movable, it will be seen that as division advances the outer 

 end swings inward toward the centre of the formative pole, and 

 at the same time comes to lie at a considerably higher level, 

 Figs. 14 and 15. As the four cells of the second quartette 

 are being cut off from the macromeres, they rotate in an anti- 

 clockwise direction until they occupy the furrows between the 

 macromeres, and by this rotation they turn the cells of the first 

 quartette back to their original positions over the centre of 



