420 TREADWELL. [Vol. XVII. 



and 2CI.I.2 (cpi'" and cpa'") have elongated considerably, their 

 nuclei have become swollen and contain prominent nucleoli, 

 and the whole cell stains with difficulty, — agreeing in this 

 respect with the cells of the primary prototroch, — the small 

 cell 2CI.I.I remains very small and with a very deeply stain- 

 ing nucleus. Later, as a result of the shifting of cell areas, 

 this small cell is shoved out of the prototroch ring, and, I 

 believe, forms a part of the ectoderm of the subumbrella, 

 though its later history is difficult to follow. This process 

 occurs in all three quadrants. Compare PI. XL, Fig. 56, an 

 embryo of 12 hours 35 minutes, with PI. XXXIX, Fig. 48, 

 an embryo of 24 hours (2bi.i.i and 2Ci.i.i). These figures 

 show two stages in the shoving of this small cell out of the 

 prototroch ring. Later embryos show it lying entirely below 

 the prototroch. The other two cells form a part of the com- 

 pleted prototroch, though they acquire their cilia very late. 

 In the stage represented in PI. XXXIX, Fig. 48, they are 

 still without cilia. A little later they push up into the proto- 

 troch ring and all the cells elongate still more, this latter 

 process coinciding with the closure of the dorsal interruption. 

 They stain very poorly, so that their outlines are impossible 

 to follow in later stages. 



The other divisions of the second quartette are as indicated 

 in PI. XXXVIII, Fig. 36; PI. XXXIX, Figs. 41, 42, 46; 

 PI. XL, Figs. 50, 5 1 ; and in the table, p. 438,^ One cell is, how- 

 ever, of especial interest. This is the cell 2b2.2.i shown in 

 PI. XXXIX, Fig. 41. Here it is seen at some distance from 

 the edge of the blastopore. It divides once only. (See the 

 products of the division in PI. XL, Fig, 51.) While the corre- 

 sponding cell in quadrant A divides vertically and equally 

 (PI. XXXIX, Fig. 41), this division is horizontal and unequal, 

 a small cell being budded off dorsally. The ventral product, 

 2b2.2.i.2, does not divide again at the surface. Gradually the 

 cells between it and the blastopore, — descendants of 2b2.2.i, 

 — invaginate to form a part of the stomodaeum, and this 



1 In PI. XXXIX, Fig. 46, I have indicated by the numeral on each cell the 

 share which the second and third quartettes take in the composition of the ven- 

 tral ectoderm. 



