No. 3] PODARKE OBSCURA VERRILL. 425 



asymmetrical divisions, and partly due to shiftings of positions 

 which the cells undergo. In PI. XXXIX, Fig. 42, for example, 

 the cells Xi.i.i.i and X2.1.1 are seen to lie close against 

 the prototroch cells in C and D quadrants respectively. Just 

 to the left of X2.1.1 are two small cells, 2a2.i.i and 2a2.i.2 

 (PI. XXXIX, Fig. 43). When the cell areas shift so that the 

 prototroch cells take their final position, I believe that the 

 cell 2a2.i.i is pushed up between X2.1.1+ and the proto- 

 troch (PI. XXXIX, Figs. 44 and 45), while, on the other hand, 

 the cell X1.1.11.+ is pushed up between the prototroch cells 

 (PI. XXXIX, Fig. 45 ; PI. XL, Fig. 56). The result of this 

 movement is that on the left side the X-cells occupy a broader 

 area, when seen from below (PI. XXXIX, Fig. 48), than do 

 those of the right of the median plane. In much later stages 

 (PI. XL, Figs. 57 and 58) a small cell lies wedged in between 

 the cells of the prototroch in the C quadrant. This cell, I 

 believe, is a descendant of Xi.i.i.i, which has been separated 

 from the X-group by the migrating cells from the upper hemi- 

 sphere. In later stages the large X3.2 divides symmetrically 

 (PI. XL, Fig. 58), but I have been able to discover no other 

 symmetrical divisions. Neither could I find any divisions, such 

 as Mead has described in Amphitrite, which, though themselves 

 asymmetrical, had direct reference to the symmetry of the 

 trochophore. 



TJie Third Quartette. — At the 64-cell stage these are eight in 

 number, two in each quadrant. We have already seen that the 

 next division establishes bilateral symmetry at the upper pole. 

 Symmetry also appears at the same time at the lower pole as a 

 result of the peculiar division of two of the third group and one 

 of the fourth. Leaving the latter for the present with merely 

 the remark that it is 4d and contains the mesoderm, let us 

 notice the division of the third quartette cells (PI. XXXVIII, 

 Figs. 26 and 27). Here 3d2 and 3C2, lying one on either side of 

 4d, are dividing in a bilaterally symmetrical fashion. (The sec- 

 ond line of cleavage in the specimen from which PL XXXVIII, 

 Fig. 27, was drawn had changed its direction. This is rare in 

 this stage and I have never found it in earlier stages before 

 X1.2 is formed.) The outer product of the division in both 



