98 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



6. THIRD CLEAVAGE. EIGHT CELLS. 



The third cleavage is essentially equatorial. The spindle figures 

 arrange themselves approximately parallel with the chief axis, and 

 therefore nearly perpendicular to the spindles of the preceding cleav- 

 ages. The spindle in the median anterior cell (& 8 ) is somewhat excep- 

 tional, in that it is more or less inclined toward the horizontal plane 

 (Plate 4, Fig. 36). The spindle in the yolk-cell d* is generally more 

 nearly parallel to the chief axis. The cells a 8 , b 8 and c a often complete 

 their division in advance of the yolk cell (Plate 11, Fig. 103). . Some- 

 times the spindle in the yolk-cell is just forming as the other cells 

 divide, but the yolk-cell completes the cleavage while the other cells 

 remain in the " resting" condition. Stages with five, six, or seven 

 cells are seen when examining living ova, but after preparation of such 

 ova the nuclei of some cells are found to be retarded in the third divis- 

 ion. Such variations in the rhythm of cleavage are not uncommon in the 

 synchronously cleaving ova of other animals. The normal " resting " 

 stage following the third cleavage in Lepas is composed of eight cells as 

 invariably as if the cleavage were perfectly synchronous in all of the 

 cells. 



The positions of the cells which result from the third cleavage are 

 shown in Figures 37-40 (Plates 4, 5), and 104-106 (Plate 11). The 

 three " protoplasmic " cells (a 8 , 6 8 , c 8 ) have divided equally, the yolk- 

 cell unequally. The cell (c? 4>2 ) which is cut off from the yolk-cell lies 

 in the median plane near the animal pole (Fig. 37). This is the 

 third micromere. The cells resulting from the division of a 8 occupy the 

 left side, and are symmetrical with those derived from c 8 , which occupy 

 the right side of the egg (Fig. 37). The cell b* has given rise to two 

 cells lying in the median plane, one (b 4 - 1 ) near the yolk-cell at the 

 vegetative pole, the other (6 4 * 2 ) at the anterior end of the egg (Figs. 

 38, 40). 



The seven " protoplasmic " cells have now begun to form the blasto- 

 derm (Plate 8, Fig. 66), which will later enclose the yolk-entoblast. 

 A very small space, which is the cleavage cavity (cav. sg., Fig. 66), 

 is often seen in sections, but it soon becomes filled with yolk, by the 

 ingrowth of the yolk-cell. 



The bilaterality in the arrangement of cells was indicated in the stage 

 with four cells ; it is well marked in the stage with eight. The charac- 

 teristic arrangement of the cells, as shown in Figures 3740, is visible 

 in the great majority of living or prepared ova, if they are properly 



