118 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



and vegetal-cells can be ascertained in figure 9, a 

 meridional section of stage 6. 



The extent of the blastocoel can also be seen in 

 figure 9. The roof of the blastocoel is composed 

 of a single layer of relatively large animal cells 

 whereas large vegetal cells extending from the 

 vegetative pole to the blastocoel constitute the 

 floor. The end-point of stage 6 is reached when 

 the embryo has 32 cells. 



Figure 9. — Meridional section of stage G. 



Stage 7: Sixty-four cells (fig. 10) hours 19-24 



Size: 1.0 millimeter. 



Cleavage: Indeterminate. 



Stage 7 is considered to have begun when the 

 embryo has more than 32 cells. The cleavage 

 continues indeterminable in this and later stages. 

 Recognition of the stage becomes a matter of cell 

 counts and comparison of cell sizes with those of 

 the preceding and succeeding stages. A cursory 

 count of the animal cells is made to get an approxi- 

 mate estimate of the stage to which the embryo 

 should be assigned; the final assignment is based 

 on a combination of cell size and cell count. As 

 cell size diminishes the contour of the embryo 

 becomes smoother. This change can be seen in 

 a comparison of figures 8 and 10. The animal cells 

 are still smaller than the vegetal cells, which have 



Figure 10. — Polar and lateral views of beginning stage 7, 

 sixty-four cells. 



since divided and are now only about twice the 

 size of the animal cells. 



The end-point of stage 7 is reached when the 

 embryo has more than 64 cells. Division of all 

 animal cells along with division of the vegetal cells 

 can be taken as an approximate end-point. 



Stage 8: Full blastula (fig. 11) hours 24-64 



Size: 1.0 millimeter, increasing to 1.2-1.4 milli- 

 meters. 



Cleavage : Furrows seen on individual cells. 



Animal hemisphere: Becomes translucent. 



Blastocoel: Visible through animal cells. 



Wlien the animal pole cells of stage 7 undergo 

 further cell division as evidenced by further reduc- 

 tion in cell size and the appearance of cleavage 

 furrows, the embryo is considered to have entered 

 stage 8. The most striking external feature of this 

 early phase of the stage is its contours (compare 

 to stage 7) which become progressively smoother 

 as the number of cell divisions increases and cell 

 size decreases. 



In the final phase of this stage the volume of the 

 embryo increases by some 3 to 5 times. Measure- 

 ments of living specimens made withm a cali- 

 brated ocular micrometer at the initiation of and 



