100 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Summary of the Third Cleavage. 
The spindles for the third cleavage are essentially perpendicular to 
those of the first two cleavages, the cleavage being practically equatorial. 
The three cells a*, 6° and c® divide equally and synchronously. The 
yolk-cell d°, which is often slightly retarded, divides unequally, the 
smaller, more protoplasmic, product (d*) of this division, being the 
third and last micromere containing ectoblast which is separated from 
the yolk-macromere. 
The yolk-cell (d*:") is now mes-entoblastic, and bilaterality in cleay- 
age is well marked. 
The arrangement of the cells of this stage is definite and constant. 
The second polar cell is crowded into the cleavage cavity during the 
third cleavage. 
7. Fourta CLEAVAGE. SIXTEEN CELLS. 
The mitotic spindles for the fourth cleavage, shown in Figures 39, 40 
(Plate 5), and 104-106 (Plate 11), have a well-marked bilateral arrange- 
ment. The cell 5*?, at the anterior end of the egg, and also the cell 
d‘? have their spindles perpendicular to the sagittal plane of the future 
embryo, and their cleavage planes coincide with that plane. In the 
yolk-cell d*:1 the mitotic spindle approaches parallelism with the chief 
axis, as in the third cleavage. In all the other cells the spindles are 
parallel with the long axis of the egg. 
The seven “protoplasmic” cells divide as a rule equally and quite 
synchronously. Division of the yolk-cell d** is delayed more than in 
the preceding cleavage, but is completed while the fourteen “ protoplas- 
mic” cells are in the “resting” phase following division (Plate 5, 
Fig. 41; Plate 8, Fig. 67; Plate 11, Fig. 108). The stage with all 
cells in the “resting” phase is composed of sixteen cells (Figs. 42, 43). 
The yolk-cell, as in the preceding divisions, has divided unequally, and 
the smaller, “ protoplasmic” cell (d**) thus formed lies in the median 
plane on the dorsal side of the embryo (animal pole) and immediately 
posterior to the cells d** and d*8, which have resulted from the division 
of d‘:?, the third micromere (Figs. 42, 44, 45, 68). This celk (d*’), 
formed by division of the yolk-cell d*’ in the fourth cleavage, is the 
primary mesoblust, as will appear from the subsequent history of its 
descendants, which sink beneath the blastoderm in a later stage. The 
yolk-cell d*-1 is now purely entodlastic. The cells a®?, 0-7, and ¢e°?, which 
touch the yolk-cell on the anterior and lateral boundaries of its uncoy- 
—— 
