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 8 , b 3 and c 3 divide equally and synchronously. The 

 yolk-cell d s , which is often slightly retarded, divides unequally, the 

 smaller, more protoplasmic, product (c? 4 - 2 ) of this division, being the 

 third and last microinere containing ectoblast which is separated from 

 the yolk-macromere. 



The yolk-cell (d 4 - 1 ) is now mes-entoblastic, and bilaterality in cleav- 

 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. FOURTH 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 & 4 * 2 , at the anterior end of the egg, and also the cell 

 d 4 ' 2 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 4 ' 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 4 - 1 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 (e? 6>2 ) thus formed lies in the median 

 plane on the dorsal side of the embryo (animal .pole) and immediately 

 posterior to the cells d 5 - 4 and d 6 - 8 , which have resulted from the division 

 of d 4 -*, the third micromere (Figs. 42, 44, 45, 68). This cell (d 5 - 2 ), 

 formed by division of the yolk-cell d 4 - 1 in the fourth cleavage, is the 

 primary mesoblast, as will appear from the subsequent history of its 

 descendants, which sink beneath the blastoderm in a later stage. The 

 yolk-cell d 6 - 1 is now purely entoUastic. The cells a 6 - 2 , J 5 ' 2 , and c 5 - 2 , which 

 touch the yolk-cell on the anterior and lateral boundaries of its uncov- 



