346 PROCEEDINGS OF THE ACADEMY OF [April, 
after the formation of the basals occupy positions just peripheral to 
them and slightly to the left. These four small second quartet ele- 
ments are the ‘‘tip”’ cells of the cross, 2a"-2d", and together with the 
basals and apicals form the ectoblastic cross. 
From the time of its formation and until a late period of cleavage 
the cross of Fiona is a distinctly dexiotropic structure, the apicals of the 
four arms lying to the right of their respective tips. The cross is thus 
at the time of its formation (fig. 23) composed of twelve cells, of which 
the apicals are the central, is radially symmetrical and its anterior and 
posterior arms lie very near to, if not exactly in, the median plane of 
the future embryo. In the future history of this structure the tip 
cells will for convenience be described in connection with the rest of the 
cross, since they are so closely connected with it. 
Before further cleavage occurs in the first quartet the second and 
third quartets and the macromeres show marked karyokinetie activity, 
the number of cells in the egg having increased to nearly sixty. The 
basal cells and the turret cells or trochoblasts then divide simultaneously 
(fig. 33), though considerable variation in time occurs in different eggs 
and in different quadrants, it being, however, universally observed 
that 1d” divides last of the basals. It may be noted in this connection 
that in all species of Crepidula examined except C. adunca the division 
in the basal cell of the posterior arm is delayed for a much longer period. 
The direction of cleavage of the basals 1d” and 1b” is leotropic and 
so alternating with the last, those of the other two doubtful; la” 
usually shows a leeotropic to radial position of spindle, while in 1c” 
variations are present all the way from lzotropic to dexiotropic. After 
examining a large number of eggs the occurrence of this irregularity 
was more strongly confirmed, and it thus appears that in this cell, 
1c”, there is a strong tendency, more marked in some cases than in 
others, toward non-alternation with resulting bilaterality of cleavage 
in relation to its opposite cell, la”. In Crepidula, Planorbis and Neri- 
tina the cleavage of all these basal cells is non-alternating, while in 
Umbrella it is regularly alternating. 
In Fiona it would appear that we have an intermediate condition in 
which, though regular alternation is found in the anterior and posterior 
basal cells, the two lateral, particularly 1c’, show a tendency toward 
non-alternation under the influence of approaching bilaterality. It 
is just at this time that the first distinctly bilateral cleavages occur in 
two cells of the third quartet in the two posterior quadrants, 3d’ and 
3c! (figs. 31, 32), and this suggestion of bilateral divisions of the cross 
may be correlated with them. However, the influence toward bilater- 
