400 GEORGE W. TANNREUTHER 



form a cavity, which later is occupied by the large cell D. Before 

 the fifth cleavage is complete, the anterior end of the cell D is 

 partially covered by the cleavage cells immediately in front of it 

 (figs. 45 to 50). 



Sixth cleavage: After the formation of the small cell d^, the 

 cell d^-i-^ (fig. 51) divides. Next a small cell d^ is formed from 

 D. The first cell of the sixth cleavage to divide is d^-^ etc. The 

 sequence of cell formation in the sixth cleavage is similar to that 

 of the fifth, the derivatives of the D quadrant dividing first, then 

 those of C, B, and A. The cell lineage of any one quadrant can 

 be followed indefinitely. Figure 51 represents the beginning of 

 the sixth cleavage. From this stage the surface cells will not 

 be labeled, as it does not contribute to the understanding of the 

 further development. The sixth cleavage doubles the number of 

 cleavage cells on the surface. It does not increase the number 

 of rows in each quadrant, but the number of cells in each row is 

 doubled. The embryo at the end of the sixth cleavage is com- 

 posed of the following cells: the D quadrant contains two rows 

 of six cells each, and d^ and d^ The C, B, and A quadrants 

 each contain two rows of eight cells each. With the large 

 entoderm cell D, there are thus sixty-three cells in all. 



3. Gastrulation 



Gastrulation, which begins during the close of the fifth cleavage, 

 adheres more strictly to the epibolic type. Immediately after 

 the formation of d^, the surrounding cells at the anterior end of 

 D begin to extend over its surface (fig. 48). The embryo when 

 viewed from the posterior end (figs. 52 and 53) shows the position 

 of the surface cells with reference to D. The spindle indicates 

 the direction in which d* is formed. Figures 52 and 54, later 

 stages during the sixth cleavage, show the method of overgrowth 

 on the surface of D (E). Gastrulation is a double process; while 

 the surface cells are extending posteriorly over the surface of E, 

 the large cell itself is migrating into the interior, a result of the 

 pressure of the surrounding cells, and the cavity within the 

 embryo, which began its formation before the sixth cleavage 

 started. 



