420 



INVEETEBEATA 



CHAP. 



Here the first spindle is formed, almost transverse to the long axis of 

 the egg, and the first cleavage plane is consequently longitudinal and 

 divides the egg into two cells, a larger, denominated I, and a smaller 

 one called A, lying side by side. The polar body lies at one end of 

 this furrow, which, since it is near the pole where the polar body was 

 given off, we shall call the animal or upper pole of the egg. 



With the rotation, however, of the polar body, which has just 

 been described, the cleavage plane separating the first two blasto- 



FlG. 331. — Further stages in the segmentation of the egg of Oallidina russeola. 

 (After Zelinka. ) 



A, 5-cell stage seen from the ventral surface. B, 6-cell stage seen from the right side. 0, 9-cell 

 stage seen from tlie left side. D, 10-celI stage seen from the ventral surface. 



meres also rotates, so that from a position almost parallel to the long 

 axis of the egg it takes up a position almost transverse to this 

 axis. In consequence of this rotation the smaller cell A swings round 

 until it occupies the posterior pole of the egg. Next a small cell is cut 

 off by a longitudinal cleavage plane from the right side of the larger 

 cell. This cell is denominated II. It slips down along the right 

 side to the posterior pole of the egg. Then A divides into two cells, 

 a and h ; h comes to occupy the posterior pole of the egg, whilst a and 

 II come to occupy opposite sides of the egg. In this way a character- 

 istic lozenge-shaped 4-cell stage is formed (Fig. 330). 



