356 PROCEEDINGS OF THE ACADEMY OF [April, 
quadrant cells of Fiona. Moreover, these cells, 3ct and 3d', divide 
again before the anterior ones as in Fiona, and these cleavages are the 
first bilateral divisions described. It would appear from Heymons’ 
figures that the two cells next the median plain lie higher than the 
outer, and this is the condition found in Fiona. If such be the case, 
these two forms stand in contradistinction to Crepidula, in which the 
median pair are the lower. The cells 3c", 3d" are the protoblasts 
of Heymons’ excretory cells, and it will be seen later that 3c" serves 
a similar purpose in Fiona. It is interesting to note that Conklin says 
of 3c“ and 3d" that they are “large and clear” and “have the same 
characteristics in Crepidula’’, though he does not know their fate. 
Heymons describes divisions at a later stage in the anterior quadrants, 
while in the posterior 3c" and 3c”, 3d" and 3d” give rise by horizontal 
divisions to small cells which lie next to 3c? and 3d?—these latter in 
exact correspondence with Fiona. 
Of this quartet Holmes says of Planorbis; “The first cleavage forms 
a transition from the spiral to the bilateral type, and subsequent 
cleavages show a bilateral character in a more marked degree. 
At nearly the same time the lower pair of cells in the two anterior 
quadrants and the upper pair of cells in the posterior quadrants divide 
in a nearly horizontal direction into equal moieties. Later the upper 
pair of cells in the anterior quadrants divide in the same direction as 
the lower pair. The lower pair of cells in the two posterior quad- 
rants remain undivided until a much later stage’. These divisions 
closely follow those of Fiona, and the same may be said of subse- 
quent ones. - 
In Aplysia (Carazzi) the two third quartet cells of each anterior 
quadrant divide into equal moieties, while in the posterior quadrants 
small cells are given off toward the vegetative pole; the same is true 
of Fiona. At the next divisions of 3c' and 3d? ‘‘si dividono con fusi 
transversali, cioé con divisione bilaterale,” while 3at and 3b* remain at 
rest. Viguier (1898) for Tethys describes the initial division of all 
the four quartet cells as “suivant des plans sensiblement radiaux”’, 
the resulting two cells in each quadrant being equal. Later cleavages 
of this quartet in Fiona will be considered under the discussion of 
gastrulation and secondary mesoderm formation, Bilaterality appears 
late in the cleavage of Trochus. The first divisions of this nature do 
not occur until the ninety-seven-cell stage, and are concerned with the 
cells 2c and 2a". This is the first violation of Sachs-Hertwig’s law 
of alternatingly perpendicular cleavages. The cleavages of the third 
quartet are very tardy in this Prosobranch, for when there are as many 
