350 PROCEEDINGS OF THE ACADEMY OF [April, 
of all the cells accurately though, after lateral extension has occurred, 
the structure may be demarkated from the trochoblasts and underlying 
second quartet cells. In fig. 75 its strueture and probably cell deri- 
vation may be seen. Holmes finds for Planorbis that the tip cells 
divide in a transverse direction first, while in Crepidula the middle 
cells are the first to cleave. The tips appear to divide last in Fiona. 
In the posterior arms after the first transverse division most of the cells 
divide obliquely across the arms, and in this way the arm becomes longer 
than the other three. While the cross is increasing in lateral extension 
the outer turret cells of all quadrants divide, so that the four groups 
each consist of four cells of equal size (fig. 75) lying in the angles 
formed by the arms of the cross. 
The apical pole of the egg at this period shows a slight depression 
in the region of the rosette series. It is but transient and disappears 
with the elongation of the gastrula. A similar depression has been 
observed in Neritina, Crepidula and Trochus. Whether the structure 
is normal in Fiona is yet doubtful. Robert insists that such is the case 
with Trochus. 
The entire formation of the cross of Trochus is peculiar. The basals 
have arisen and divided before the tips appear, and this division of 
the basals is so directly leotropic as to be practically transverse. At 
the next cleavage these two cells form an oblong group of four in each 
arm. The tips which lie peripherally to these. groups next divide, 
the cleavages of 2a! and 2c being bilateral, the first of this nature to 
occur in the egg. 
From the cases cited above of the manner of formation of the 
ectoblastie cross of Mollusks, it will be seen that this characteristic 
structure shows great diversity of details throughout the group, 
though fundamental similarity is evident. Some of the probable 
causes of such variation are (1) varying amounts of yolk, leading 
to early lateral extension of the arms in those forms possessing 
yolk-ladened entomeres, and (2) differences in the manner and rate 
of development of the trochoblasts, correlated with the later structure 
and functional importance of the locomotor organ to which they 
largely give rise. The radial arrangement of blastomeres around the 
apical pole of the cleaving egg is primarily the result of successively 
alternating spiral cleavages, and a similar arrangement may be expected 
in eggs which exhibit this mode of division. A definitely marked cross 
does not always arise from such an arrangement of blastomeres, as, 
for example, in Polyclad cleavage, so that this but suffices as a partial 
explanation. Regarding the form of the crosses of Mollusks and 
