402 HOLMES. [Vol. XVI. 



cell laeotropically ; in the posterior pair the cleavage of the right 

 cell is dexiotropic, that of the left one laeotropic. The two cells 

 in the anterior arm, i (5''''''^'', i<5'''^"'", next divide, the right cell 

 in a dexiotropic, the left in a laeotropic direction. The poste- 

 rior cells on either side resulting from this division come to lie 

 to the outside of the small, outer, intermediate cells, so that they 

 no longer lie in contact with the trochoblasts (PL XX, Fig. 42). 

 (Compare the cleavage of \b''^''^ and ib^''^" in Crepidula.) Next 

 the anterior median cell, ib^'^'"^, divides transversely, and each of 

 the daughter-cells divides bilaterally, in the same direction as 

 that of the pair of cells just described. 



At this period the general form of the cross has undergone 

 marked changes. The cells of the posterior arm have increased 

 greatly in size and become distorted in shape. The posterior 

 trochoblasts enlarge unequally, and the posterior arm may be 

 pushed either to the right or the left. The posterior tip cell 

 first increases in size ; afterward, all the cells lying in front of 

 it also enlarge. The enlargement of cells does not stop with 

 the cells of the posterior arm, but the four apical cells increase 

 in size also (PI. XX, Fig. 42). The central and anterior portions 

 of the cross are pushed forward more rapidly than the lateral 

 arms, which thus appear to be bent backwards. The darkly 

 staining portion of the cross has now the shape of a FJ with the 

 apex pointing anteriorly. The two anterior tip cells, as the 

 cross is pushed forward, come to lie more and more nearly side 

 by side, and, finally, they become arranged transversely across 

 the tip of the cross. As they were originally somewhat oblique, 

 this arrangement would very naturally result from a pressure 

 due to the forward rotation of the apical cap of cells. 



The process of enlargement of cells extends forward to the 

 cells lying in front of the apicals, and eventually forms a tract 

 of large clear cells, which separates the two halves of the cross 

 and reaches the prototroch in front. The basal cell of the ante- 

 rior arm of the cross, since it lies in the median line, takes part 

 in this enlargement of cells ; as it increases in size, it be- 

 comes pushed forward, and the cells in front of it, which lie 

 symmetrically on either side of the median axis of the arm, are 

 forced aside. The fate of this cell has been carefully traced, as 



