98 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
6. Tarrp CLEAVAGE. E1eHt 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 (6°) 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’, 4° and c® 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%, 6°, c?) have divided equally, the yolk- 
cell unequally. The cell (d*?) 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® occupy the 
left side, and are symmetrical with those derived from ce’, which occupy 
the right side of the egg (Fig. 37). The cell 6° has given rise to two 
cells lying in the median plane, one (4**) near the yolk-cell at the 
vegetative pole, the other (+?) 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. sq., 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 37-40, is visible 
in the great majority of living or prepared ova, if they are properly 
