180 TORREY 



which lead the van in AuipJiitrite, Arenicola and Podarke. I 

 have decided, accordingly, to follow Mead's ('97) nomenclature 

 and designate them as /^ and 4. The upper cell (1^1.211) 

 divides several times and its progeny also finally passes down- 

 ward and through the dorsal gap. By a dexiotropic cleavage 

 the intergirdle cell, u^'i. o. 2. (the "/" of Mead) gives rise to a 

 rudimentary cell (PL II, Fig. 14). The subsequent fate of the 

 former I have not definitely established, but it probably takes 

 part in the post-trochal migration. The only further divisions 

 of interest in the a, b and c intergirdle regions are those of the 

 cells concerned in the formation of the prototroch (i<^i. 2. 2. > 

 etc., see PI. I, Fig. 6). The cells 1^1.2.2. ^^d ir^ o. 2. divide 

 dexiotropically and nearly equally. The lower products {la 

 1.2.2.2. ^^"^d i-Tj 2. 2. 2.) later form a part of the prototroch. The 

 cell 1^1.2.2. divides in the same direction, but buds off a rudi- 

 mentary cell (i<^i. 2. 2. 1.)' The larger cell (1^1.2.2.2.) finally en- 

 ters the prototroch as in the other quadrants (PI. I, Fig. 7). 



TJie Cross. — The distal cell in each of the posterior arms of 

 the cross buds off a very minute cell upw^ard and toward the 

 median dorsal line (PL I, Fig. 4). (Exactly similar cells are 

 formed in Podarke.) They later degenerate completely. The 

 subsequent divisions in the posterior arms of the cross are all 

 directed toward the median dorsal line, so that, finally, the arms 

 come in contact along this line and thereby shove the adjacent 

 intergirdle cells through the dorsal gap in the prototroch (PL I, 

 Figs. 5, 6 and 7). At the 64-cell stage each of the anterior 

 arms of the cross consisted of two cells (PL I, Fig. 3). Equal, 

 radial divisions of the distal or *' stem " cells next take place 

 (PL I, Fig. 4). By a series of bilateral divisions each arm be- 

 comes split throughout its entire extent (PL I, Figs. 6 and 7). 

 A continuation of the lateral spreading of the arms, initiated by 

 these cleavages, results in the formation from the descendants of 

 the proximal cross cells of almost the entire upper part of the 

 umbrella of the trochophore. The lower part of the prae-trochal 

 region arises from an alternation of the intergirdle and distal 

 cross cells, except "in the dorsal region, which is almost wholly 

 occupied by the cross cells. A detailed description of all these 



