84 BIOLOGICAL LECTURES. 



tropic in the two lower blastomeres (the ones meeting in the 

 polar furrow), and frequently laeotropic in the upper ones (Fig. 9). 

 As the cleavage advances the spindles are all turned in a dexio- 

 tropic direction. All this time the remains of the spheres of 

 the preceding cleavage occupy the central angles of the cells, 

 where they are plainly undergoing disintegration (Figs. 9-1 1). 

 During and after this cleavage the cell currents rotate in a 

 dexiotropic direction in the upper cells (micromeres), and in a 

 laeotropic direction in the lower cells (macromeres) ; this move- 

 ment is manifest during cleavage in the dexiotropic lobing of 

 the cytoplasm in each cell preparatory to the formation of the 

 micromeres, and in the obliquity of the cell plate to the axis of 

 the spindle (Fig. 11); after cleavage it appears in the rotation 

 of the centrospheres and nuclei. Even after the third cleavage 

 is completed the sphere substance of the second cleavage can 

 be seen in the inner angles of the micromeres, where it grad- 

 ually disintegrates and passes into the cytoplasm. The dexio- 

 tropic rotation of the protoplasm in the micromeres continues 

 until the daughter-nuclei are carried from the left to the right 

 side of the cells; at the same time the substance of the macro- 

 meres rotates to the left, until the nuclei and spheres are 

 brought into contact with the cell membranes at the left of 

 these cells (Fig. 12). Here the centrosomes and central spindles 

 arise from the inner spheres of the centrospheres, and the laeo- 

 tropic movement of the cytoplasm continues until the spindles, 

 which at first are equatorial, become decidedly laeotropic in 

 position, and the second quartette of micromeres is given off in 

 a laeotropic direction. As the whole of the sphere substance 

 of the second cleavage goes into the first quartette of micro- 

 meres, so all the sphere substance of the third cleavage remain- 

 ing in the macromeres goes into the second quartette, where it 

 rapidly disintegrates and disappears. The contents of the second 

 quartette move in a laeotropic direction until the centrosphere 

 is carried from the extreme left to the extreme right of each 

 cell ; at the same time the substance of the macromeres moves 

 in a dexiotropic direction until the centrospheres and the nuclei 

 are carried to the right side of each cell, and in this position 

 the third quartette is given off in a dexiotropic direction (Figs, 



