116 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 4. — External views of cleavage extremes of stage 2. 



the cell, the ceU membrane expands in advance of 

 the progressing furrow. 



Stage 3: Four cells (flg. 5) hours 8-11 



Cellular areas : Visible in four daughter cells. 

 Size: 1.0 ±0.2 millimeters. 



Figure 5. — Various external views of stage 3, four cells. 



Cleavage: Second furrow appears: peaks are 

 less prominent than in first division. Holoblastic. 

 Completed within 3 hours. 



Despite cellular division the three distinct topo- 

 graphical areas are still discernible in each of the 

 four cells. 



Stage 3 begins with the advent of the second 

 cleavage furrow and dimple which starts in much 

 the same manner as the first; the furrow and 

 dimple are observed at the animal pole to either 

 side of the first cleavage furrow and progress 

 meridionally at right angles to the first cleavage 

 furrow. As in stage 2, the expanding cell mem- 

 branes are observed during cytokinesis. 



The end-point of stage 3 is the appearance of 

 third cleavage furrow and dimple. 



Stage 4: Eight cells (flg. 6) hours 10-15 



Cellular areas: Visible in meridional type 

 cleavage. 



Size: About 1.0 millimeter for equatorial divi- 

 sion. Meridional division increases equatorial 

 diameter and shortens meridional diameter. 



Cleavage: Two types; meridional or equatorial. 

 Holoblastic. Completed within 2 hours. 



Stage 4 begins with the appearance of the third 

 cleavage furrow, which may be either a double 



Figure 6. — Polar views of meridional and equatorial cleavages forming stage 4, eight cells. 



