No. 3-] POD ARK E OBSCURA VERRILL. 42 I 



cell, 2b2.2.i.2, approaches closer and closer to the blastoporic 

 margin, and finally invaginates. (See PI. XXXIX, Figs. 43, 

 44; PI. XL, Figs. 49, 50, 51, 52, 54, 55.) In the stage repre- 

 sented in PI. XXXIX, Fig. 47, it lies underneath the surface 

 in the wall of the stomodaeum, where its nucleus is indicated 

 by the dotted line. It later divides, but I have not been able to 

 follow its subsequent history farther than to say that it forms 

 a part of the stomodaeal wall. 



The history of the second quartette cells in the dorsal quad- 

 rant is of especial interest for purposes of comparison, since 

 in many annelids and mollusks they give rise to a large part 

 or all of the ectoderm of the body, and in Amphitrite and 

 Arenicola, at least, the paratroch of the larva is formed from 

 descendants of this group. I shall follow Wilson and Mead 

 in giving to these cells the especial name "X." For the sake 

 of clearness I have enclosed the X-cells in a heavy outline. 



The first divisions of X have already been described, 

 the small X1.2 arising in the 64-cell stage. (See PI. XXXVIII, 

 Fig. 26, where I have labelled the figure to correspond with 

 Mead's nomenclature for the X-cells.) The next divisions 

 occur in X^.i and X^ (PI. XXXVIII, Fig. 30, and PI. XXXIX, 

 Fig. 39), and these are soon followed by a division of X, (PI. 

 XXXVIII, Figs. 31 and 32), where the completion of this 

 division is shown. Xi.i.i next divides (PI. XXXIX, Fig. 39), 

 and there results a larger cell above, Xi.i.i.i, and a smaller, 

 X1.1.1.2. below (PI. XXXIX, Fig. 40). At nearly the same 

 time Xx.2 divides (PI. XXXVIII, Fig. 35, and PI. XXXIX, 

 Fig. 39). In PI. XXXIX, Fig. 40, is shown the division 

 of X2.1, and X2.2, and X1.1.2. PI. XXXIX, Fig. 42, shows the 

 division completed. Xi.i.i.i next divides and at about the 

 same time division occurs in X2.1.1. These divisions are shown 

 as completed in PI. XXXIX, Fig. 42. These cells with their 

 descendants now become diflJicult to distinguish from the cells 

 of the third quartette near them, and as nothing of importance 

 was to be gained from such a study, I have not attempted to 

 carry them farther. X1.1.2.1 and Xi. 1.2.2 next divide equally 

 (PI. XXXIX, Fig. 43), and X2.2.1 (PI. XXXIX, Fig. 44), very 

 unequally, the latter budding off a small cell to the left of 



