238 MEAD. [Vol. XIII. 



clined at an angle of 45° to the median plane, while the rosette 

 lies in the middle of the cross (Figs. 18, 24, 29, cross, blue). 

 The rosette cells later divide obliquely to the right, thus con- 

 tinuing in the alternating cleavage (Fig. 30). The intermediate 

 cells, those in the angles between the arms of the cross, all 

 divide in the same direction — right oblique — though not all 

 at the same time. As a general rule, the larger ones divide 

 first, the smaller last (Fig. 30). 



The sixteen cells of the primary prototroch never divide again, 

 but are flattened down so as to present a very even surface, 

 and soon become covered with cilia. 



The succeeding divisions of the cross cells are of great inter- 

 est. They are exactly bilateral, so that the divisions on one side 

 are the mirrored image of those on the other (Figs. 30-36). 

 First, the distal cells in the dorsal arms of the cross divide, 

 ^1.3.2^ ^1.3.2 . 1^^^^ before these divisions are completed, spindles 

 appear in the corresponding cells of the ventral arms, «''^^ (5'"^'^ 

 and in the proximal cells of the dorsal arms, and soon also in 

 the proximal cells of the ventral arms, ^''^'^ and U"^'^. When 

 all these divisions are completed, each arm of the cross has 

 three cells in a row and an extra one at the base. The cross is 

 not only symmetrical with respect to the TJiedian sagittal plane, 

 btit neaj'ly so with respect to a plane at right angles to this. So 

 far, and even farther, the cross in Amphitrite is exactly com- 

 parable to that in Nereis (cf. Wilson,^ PI. XVI, Fig. 40). The 

 middle cells in the dorsal arms, corresponding to the " nephro- 

 blasts " in Nereis, never divide again, and, except for a small 

 area left at the surface, are covered over by the surrounding 

 cells, and become the huge dorsal ttmbrellar mticotis glands 

 (Figs. 31-37, text Figs. XI-XVII, gl.l. and gl.r., p. 257). The 

 striking similarity between the dorsal and ventral arms of the 

 cross at once suggests that the middle cells in the ventral arms 

 have a destiny similar to that of gl.r. and gl.l. — Unlike the 

 dorsal cells, they each divide once meridionally, but this I 

 think is their last division (Fig. 33). They are soon partially 

 covered over like the corresponding cells in the dorsal arms. 

 Since two pairs of unicellular mucous glands occupy a position 

 in the later larva exactly corresponding to these two pairs of 



