1904.] NATURAL SCIENCES OF PHILADELPHIA. 347 
ality must be very slight, as the radial symmetry of the upper pole is 
not disturbed to any appreciable degree. 
By the divisions of the basal cells above described each arm of the 
cross is composed of four cells—an outer tip cell (2a"—2d"), next to it 
the ‘‘middle”’ cell (1a??-1d?”), an inner “‘ basal” cell (1a!4-1d!"), which 
is larger than its sister middle cell, and an apical (1a4—1d"). 
Synchronously with the cleavage of the basals occurs that of the 
turrets, the cell of this series in each quadrant dividing into two of 
nearly equal size, the outer being the smaller. All divisions are 
dexiotropic and alternating with those by which these cells arose 
(fig. 33). 
Comparing the cleavage of the turrets with conditions found in other 
forms, it will be noted that considerable variation exists. While in 
Fiona these cells divide when there are about sixty blastomeres in 
the whole egg, in Umbrella (Heymons) approximately seventy are 
present; like Fiona all four turrets divide at relatively the same time. 
In Crepidula the anterior trochoblasts do not divide until there are 
over one hundred cells in the egg, and Conklin states that he believes 
the posterior ones never divide. The trochoblasts of Trochus (Robert) 
arise very early, at the sixteen-cell stage, and have all divided when 
there are thirty-two cells present. In Planorbis Holmes finds them in 
division at about forty cells, and Limaz (Kofoid) shows a similar con- 
dition. In Unio (Lillie) there are about fifty cells, while in Jschno- 
chiton (Heath) but thirty-two, when the ‘primary trochoblasts” of the 
latter form divide. Thus Fiona appears to occupy an intermediate 
position in relation to these and other molluscan forms in which the 
time of cleavage of these cells has been determined. 
Division next occurs in the cross at a stage of about eighty-four 
cells and results in the division of the apicals into eight small cells, 
of which those lying centrally form the “apical rosettes” (1a‘4—1d"), 
_while the outer series are the “peripheral rosettes” (la™*-1d"™) of 
Conklin. Direction of cleavage is leotropic, and of the resulting cells the 
outer are the larger (Pl. XX VII, fig. 53). Shortly after the rosette series 
are established the basal cells of all arms divide again, the posterior 
one last. In the anterior quadrant the spindle and resulting cells, 
1b?" and 1b’, lie radially in the lateral arms, the division of 1c’! is 
leotropic, that of 1a’*! dexiotropic, again showing bilateral influence, 
while in 1d! the spindle is so strongly turned in leotropic direction 
that the resulting cells lie transversely across the posterior arm (figs. 
56, 62). While this last cleavage of the basals is being accomplished 
a similar process is seen in the four inner trochoblasts (1a7"-1d”), result- 
