No. 3] PODARKE OBSCURA VERRILL. 419 



tuft of apical cilia which from now on is a prominent feature of 

 the larva. I think that all of the eight cells become ciliated. 



TJie Sccojid Quartette. — At the 64-cell stage there are sixteen 

 of these cells, four in each quadrant, and a difference between 

 the quadrants has shown itself by the peculiar division of one 

 cell, leading to the formation of X1.2, already mentioned. 

 Since the fate of the second group of micromeres in the 

 dorsal quadrant is different from that in the others, it will be 

 advantageous to consider it by itself, and turn first to the 

 second quartette in quadrants A, B, and C. 



The first division concerns the upper dextral and lower 

 sinistral cell (PI. XXXVIII, Fig. 29). This is followed some- 

 what later by a division of the upper sinistral and lower 

 dextral cells (PI. XXXIX, Fig. 37). These divisions follow 

 the law of alternation of cleavages only in the dextral cells, 

 the sinistral dividing in precisely the same direction as their 

 preceding division. Neither can the divisions be called bi- 

 lateral, since the cells are not bilaterally arranged ; and, 

 indeed, from the 64-cell stage on, while some of the divisions 

 of cells which lie near the median plane may be bilateral, 

 the majority are not. They develop so as to produce in the 

 end an embryo with bilaterally symmetrical organs ; but data 

 concerning the precise direction of each spindle, except in so 

 far as they bear on the identification of cells, seem to me of 

 little value. Cleavages may follow the law of alternation, or 

 they may not, and neither case in itself is of any importance. 

 The divisions of the dextral cells are of interest, for by them 

 are produced three cells, which correspond to the " secondary 

 trochoblasts " of Amphitrite, the cells 23.1.1.1, 2ai.i.2, 2ai.2.i, 

 etc. (PI. XXXIX, Fig. 38). In Amphitrite, however, the 

 division of the upper cell is equal, while in Podarke it is very 

 unequal (PI. XXXIX, Figs. 37 and 38). 



In each quadrant, then, there are three cells, two large and 

 one small, corresponding in origin to the secondary trocho- 

 blasts of Amphitrite. (See PI. XXXIX, Figs. 37, 38, 41, etc., 

 api'", ap.'", 2ai.i.i, bpi'", bp2'", 2bi.i.i, cpi'", cp2"', 2Ci.i.i.) 

 Fig. 56 (PI. XL) shows the relation of these cells to the rest 

 of the prototroch : cp/", cp2'", 2Ci.i.i. While, however, 2Ci.i.i, 



